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/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 directives are processed in the following
49 order: DenyUsers, AllowUsers, DenyGroups, and finally
50 AllowGroups.
51 See PATTERNS in ssh_config(5) for more information on patterns.
53 AllowStreamLocalForwarding
55 Specifies whether StreamLocal (Unix-domain socket) forwarding is
56 permitted. The available options are yes (the default) or all to
57 allow StreamLocal forwarding, no to prevent all StreamLocal for‐
58 warding, local to allow local (from the perspective of ssh(1))
59 forwarding only or remote to allow remote forwarding only. Note
60 that disabling StreamLocal forwarding does not improve security
61 unless users are also denied shell access, as they can always
62 install their own forwarders.
63 AllowTcpForwarding
65 Specifies whether TCP forwarding is permitted. The available
66 options are yes (the default) or all to allow TCP forwarding, no
67 to prevent all TCP forwarding, local to allow local (from the
68 perspective of ssh(1)) forwarding only or remote to allow remote
69 forwarding only. Note that disabling TCP forwarding does not
70 improve security unless users are also denied shell access, as
71 they can always install their own forwarders.
72 AllowUsers
74 This keyword can be followed by a list of user name patterns,
75 separated by spaces. If specified, login is allowed only for
76 user names that match one of the patterns. Only user names are
77 valid; a numerical user ID is not recognized. By default, login
78 is allowed for all users. If the pattern takes the form
79 USER@HOST then USER and HOST are separately checked, restricting
80 logins to particular users from particular hosts. HOST criteria
81 may additionally contain addresses to match in CIDR
82 address/masklen format. The allow/deny directives are processed
83 in the following order: DenyUsers, AllowUsers, DenyGroups, and
84 finally AllowGroups.
85 See PATTERNS in ssh_config(5) for more information on patterns.
87 AuthenticationMethods
89 Specifies the authentication methods that must be successfully
90 completed for a user to be granted access. This option must be
91 followed by one or more lists of comma-separated authentication
92 method names, or by the single string any to indicate the default
93 behaviour of accepting any single authentication method. If the
94 default is overridden, then successful authentication requires
95 completion of every method in at least one of these lists.
96 For example, "publickey,password publickey,keyboard-interactive"
98 would require the user to complete public key authentication,
99 followed by either password or keyboard interactive authentica‐
100 tion. Only methods that are next in one or more lists are
101 offered at each stage, so for this example it would not be possi‐
102 ble to attempt password or keyboard-interactive authentication
103 before public key.
104 For keyboard interactive authentication it is also possible to
106 restrict authentication to a specific device by appending a colon
107 followed by the device identifier bsdauth or pam. depending on
108 the server configuration. For example,
109 "keyboard-interactive:bsdauth" would restrict keyboard interac‐
110 tive authentication to the bsdauth device.
111 If the publickey method is listed more than once, sshd(8) veri‐
113 fies that keys that have been used successfully are not reused
114 for subsequent authentications. For example,
115 "publickey,publickey" requires successful authentication using
116 two different public keys.
117 Note that each authentication method listed should also be
119 explicitly enabled in the configuration.
120 The available authentication methods are: "gssapi-with-mic",
122 "hostbased", "keyboard-interactive", "none" (used for access to
123 password-less accounts when PermitEmptyPasswords is enabled),
124 "password" and "publickey".
125 AuthorizedKeysCommand
127 Specifies a program to be used to look up the user's public keys.
128 The program must be owned by root, not writable by group or oth‐
129 ers and specified by an absolute path. Arguments to
130 AuthorizedKeysCommand accept the tokens described in the TOKENS
131 section. If no arguments are specified then the username of the
132 target user is used.
133 The program should produce on standard output zero or more lines
135 of authorized_keys output (see AUTHORIZED_KEYS in sshd(8)). If a
136 key supplied by AuthorizedKeysCommand does not successfully
137 authenticate and authorize the user then public key authentica‐
138 tion continues using the usual AuthorizedKeysFile files. By
139 default, no AuthorizedKeysCommand is run.
140 AuthorizedKeysCommandUser
142 Specifies the user under whose account the AuthorizedKeysCommand
143 is run. It is recommended to use a dedicated user that has no
144 other role on the host than running authorized keys commands. If
145 AuthorizedKeysCommand is specified but AuthorizedKeysCommandUser
146 is not, then sshd(8) will refuse to start.
147 AuthorizedKeysFile
149 Specifies the file that contains the public keys used for user
150 authentication. The format is described in the AUTHORIZED_KEYS
151 FILE FORMAT section of sshd(8). Arguments to AuthorizedKeysFile
152 accept the tokens described in the TOKENS section. After expan‐
153 sion, AuthorizedKeysFile is taken to be an absolute path or one
154 relative to the user's home directory. Multiple files may be
155 listed, separated by whitespace. Alternately this option may be
156 set to none to skip checking for user keys in files. The default
157 is ".ssh/authorized_keys .ssh/authorized_keys2".
158 AuthorizedPrincipalsCommand
160 Specifies a program to be used to generate the list of allowed
161 certificate principals as per AuthorizedPrincipalsFile. The pro‐
162 gram must be owned by root, not writable by group or others and
163 specified by an absolute path. Arguments to
164 AuthorizedPrincipalsCommand accept the tokens described in the
165 TOKENS section. If no arguments are specified then the username
166 of the target user is used.
167 The program should produce on standard output zero or more lines
169 of AuthorizedPrincipalsFile output. If either
170 AuthorizedPrincipalsCommand or AuthorizedPrincipalsFile is speci‐
171 fied, then certificates offered by the client for authentication
172 must contain a principal that is listed. By default, no
173 AuthorizedPrincipalsCommand is run.
174 AuthorizedPrincipalsCommandUser
176 Specifies the user under whose account the
177 AuthorizedPrincipalsCommand is run. It is recommended to use a
178 dedicated user that has no other role on the host than running
179 authorized principals commands. If AuthorizedPrincipalsCommand
180 is specified but AuthorizedPrincipalsCommandUser is not, then
181 sshd(8) will refuse to start.
182 AuthorizedPrincipalsFile
184 Specifies a file that lists principal names that are accepted for
185 certificate authentication. When using certificates signed by a
186 key listed in TrustedUserCAKeys, this file lists names, one of
187 which must appear in the certificate for it to be accepted for
188 authentication. Names are listed one per line preceded by key
189 options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)).
190 Empty lines and comments starting with ‘#’ are ignored.
191 Arguments to AuthorizedPrincipalsFile accept the tokens described
193 in the TOKENS section. After expansion, AuthorizedPrincipalsFile
194 is taken to be an absolute path or one relative to the user's
195 home directory. The default is none, i.e. not to use a princi‐
196 pals file – in this case, the username of the user must appear in
197 a certificate's principals list for it to be accepted.
198 Note that AuthorizedPrincipalsFile is only used when authentica‐
200 tion proceeds using a CA listed in TrustedUserCAKeys and is not
201 consulted for certification authorities trusted via
202 ~/.ssh/authorized_keys, though the principals= key option offers
203 a similar facility (see sshd(8) for details).
204 Banner The contents of the specified file are sent to the remote user
206 before authentication is allowed. If the argument is none then
207 no banner is displayed. By default, no banner is displayed.
208 CASignatureAlgorithms
210 Specifies which algorithms are allowed for signing of certifi‐
211 cates by certificate authorities (CAs). The default is:
212 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
214 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
215 Certificates signed using other algorithms will not be accepted
217 for public key or host-based authentication.
218 ChallengeResponseAuthentication
220 Specifies whether challenge-response authentication is allowed
221 (e.g. via PAM or through authentication styles supported in
222 login.conf(5)) The default is yes.
223 ChrootDirectory
225 Specifies the pathname of a directory to chroot(2) to after
226 authentication. At session startup sshd(8) checks that all com‐
227 ponents of the pathname are root-owned directories which are not
228 writable by any other user or group. After the chroot, sshd(8)
229 changes the working directory to the user's home directory.
230 Arguments to ChrootDirectory accept the tokens described in the
231 TOKENS section.
232 The ChrootDirectory must contain the necessary files and directo‐
234 ries to support the user's session. For an interactive session
235 this requires at least a shell, typically sh(1), and basic /dev
236 nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4),
237 and tty(4) devices. For file transfer sessions using SFTP no
238 additional configuration of the environment is necessary if the
239 in-process sftp-server is used, though sessions which use logging
240 may require /dev/log inside the chroot directory on some operat‐
241 ing systems (see sftp-server(8) for details).
242 For safety, it is very important that the directory hierarchy be
244 prevented from modification by other processes on the system
245 (especially those outside the jail). Misconfiguration can lead
246 to unsafe environments which sshd(8) cannot detect.
247 The default is none, indicating not to chroot(2).
249 Ciphers
251 Specifies the ciphers allowed. Multiple ciphers must be comma-
252 separated. If the specified list begins with a ‘+’ character,
253 then the specified ciphers will be appended to the default set
254 instead of replacing them. If the specified list begins with a
255 ‘-’ character, then the specified ciphers (including wildcards)
256 will be removed from the default set instead of replacing them.
257 If the specified list begins with a ‘^’ character, then the spec‐
258 ified ciphers will be placed at the head of the default set.
259 The supported ciphers are:
261 3des-cbc
263 aes128-cbc
264 aes192-cbc
265 aes256-cbc
266 aes128-ctr
267 aes192-ctr
268 aes256-ctr
269 aes128-gcm@openssh.com
270 aes256-gcm@openssh.com
271 chacha20-poly1305@openssh.com
272 The default is handled system-wide by crypto-policies(7). To see
274 the defaults and how to modify this default, see manual page
275 update-crypto-policies(8).
276 The list of available ciphers may also be obtained using "ssh -Q
278 cipher".
279 ClientAliveCountMax
281 Sets the number of client alive messages which may be sent with‐
282 out sshd(8) receiving any messages back from the client. If this
283 threshold is reached while client alive messages are being sent,
284 sshd will disconnect the client, terminating the session. It is
285 important to note that the use of client alive messages is very
286 different from TCPKeepAlive. The client alive messages are sent
287 through the encrypted channel and therefore will not be spoofa‐
288 ble. The TCP keepalive option enabled by TCPKeepAlive is spoofa‐
289 ble. The client alive mechanism is valuable when the client or
290 server depend on knowing when a connection has become unrespon‐
291 sive.
292 The default value is 3. If ClientAliveInterval is set to 15, and
294 ClientAliveCountMax is left at the default, unresponsive SSH
295 clients will be disconnected after approximately 45 seconds.
296 ClientAliveInterval
298 Sets a timeout interval in seconds after which if no data has
299 been received from the client, sshd(8) will send a message
300 through the encrypted channel to request a response from the
301 client. The default is 0, indicating that these messages will
302 not be sent to the client.
303 Compression
305 Specifies whether compression is enabled after the user has
306 authenticated successfully. The argument must be yes, delayed (a
307 legacy synonym for yes) or no. The default is yes.
308 DenyGroups
310 This keyword can be followed by a list of group name patterns,
311 separated by spaces. Login is disallowed for users whose primary
312 group or supplementary group list matches one of the patterns.
313 Only group names are valid; a numerical group ID is not recog‐
314 nized. By default, login is allowed for all groups. The
315 allow/deny directives are processed in the following order:
316 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
317 See PATTERNS in ssh_config(5) for more information on patterns.
319 DenyUsers
321 This keyword can be followed by a list of user name patterns,
322 separated by spaces. Login is disallowed for user names that
323 match one of the patterns. Only user names are valid; a numeri‐
324 cal user ID is not recognized. By default, login is allowed for
325 all users. If the pattern takes the form USER@HOST then USER and
326 HOST are separately checked, restricting logins to particular
327 users from particular hosts. HOST criteria may additionally con‐
328 tain addresses to match in CIDR address/masklen format. The
329 allow/deny directives are processed in the following order:
330 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
331 See PATTERNS in ssh_config(5) for more information on patterns.
333 DisableForwarding
335 Disables all forwarding features, including X11, ssh-agent(1),
336 TCP and StreamLocal. This option overrides all other forwarding-
337 related options and may simplify restricted configurations.
338 ExposeAuthInfo
340 Writes a temporary file containing a list of authentication meth‐
341 ods and public credentials (e.g. keys) used to authenticate the
342 user. The location of the file is exposed to the user session
343 through the SSH_USER_AUTH environment variable. The default is
344 no.
345 FingerprintHash
347 Specifies the hash algorithm used when logging key fingerprints.
348 Valid options are: md5 and sha256. The default is sha256.
349 ForceCommand
351 Forces the execution of the command specified by ForceCommand,
352 ignoring any command supplied by the client and ~/.ssh/rc if
353 present. The command is invoked by using the user's login shell
354 with the -c option. This applies to shell, command, or subsystem
355 execution. It is most useful inside a Match block. The command
356 originally supplied by the client is available in the
357 SSH_ORIGINAL_COMMAND environment variable. Specifying a command
358 of internal-sftp will force the use of an in-process SFTP server
359 that requires no support files when used with ChrootDirectory.
360 The default is none.
361 GatewayPorts
363 Specifies whether remote hosts are allowed to connect to ports
364 forwarded for the client. By default, sshd(8) binds remote port
365 forwardings to the loopback address. This prevents other remote
366 hosts from connecting to forwarded ports. GatewayPorts can be
367 used to specify that sshd should allow remote port forwardings to
368 bind to non-loopback addresses, thus allowing other hosts to con‐
369 nect. The argument may be no to force remote port forwardings to
370 be available to the local host only, yes to force remote port
371 forwardings to bind to the wildcard address, or clientspecified
372 to allow the client to select the address to which the forwarding
373 is bound. The default is no.
374 GSIAllowLimitedProxy
376 Specifies whether to accept limited proxy credentials for authen‐
377 tication. The default is no.
378 GSSAPIAuthentication
380 Specifies whether user authentication based on GSSAPI is allowed.
381 The default is yes.
382 GSSAPICleanupCredentials
384 Specifies whether to automatically destroy the user's credentials
385 cache on logout. The default is yes.
386 GSSAPICredentialsPath
388 If specified, the delegated GSSAPI credential is stored in the
389 given path, overwriting any existing credentials. Paths can be
390 specified with syntax similar to the AuthorizedKeysFile option
391 (i.e., accepting %h and %u tokens). When using this option, set‐
392 ting 'GssapiCleanupCredentials no' is recommended, so logging out
393 of one session doesn't remove the credentials in use by another
394 session of the same user. Currently only implemented for the GSI
395 mechanism.
396 GSSAPIDelegateCredentials
398 Specifies whether delegated credentials are stored in the user's
399 environment. The default is yes.
400 GSSAPIEnablek5users
402 Specifies whether to look at .k5users file for GSSAPI authentica‐
403 tion access control. Further details are described in ksu(1).
404 The default is no.
405 GSSAPIKeyExchange
407 Specifies whether key exchange based on GSSAPI is allowed. GSSAPI
408 key exchange doesn't rely on ssh keys to verify host identity.
409 The default is yes.
410 GSSAPIStrictAcceptorCheck
412 Determines whether to be strict about the identity of the GSSAPI
413 acceptor a client authenticates against. If set to yes then the
414 client must authenticate against the host service on the current
415 hostname. If set to no then the client may authenticate against
416 any service key stored in the machine's default store. This
417 facility is provided to assist with operation on multi homed
418 machines. The default is yes.
419 GSSAPIStoreCredentialsOnRekey
421 Controls whether the user's GSSAPI credentials should be updated
422 following a successful connection rekeying. This option can be
423 used to accepted renewed or updated credentials from a compatible
424 client. The default is “no”.
425 For this to work GSSAPIKeyExchange needs to be enabled in the
427 server and also used by the client.
428 GSSAPIKexAlgorithms
430 The list of key exchange algorithms that are accepted by GSSAPI
431 key exchange. Possible values are
432 gss-gex-sha1-,
434 gss-group1-sha1-,
435 gss-group14-sha1-,
436 gss-group14-sha256-,
437 gss-group16-sha512-,
438 gss-nistp256-sha256-,
439 gss-curve25519-sha256-
440 The default is handled system-wide by crypto-policies(7). To see
442 the defaults and how to modify this default, see manual page
443 update-crypto-policies(8).
444 HostbasedAcceptedKeyTypes
446 Specifies the key types that will be accepted for hostbased
447 authentication as a list of comma-separated patterns. Alter‐
448 nately if the specified list begins with a ‘+’ character, then
449 the specified key types will be appended to the default set
450 instead of replacing them. If the specified list begins with a
451 ‘-’ character, then the specified key types (including wildcards)
452 will be removed from the default set instead of replacing them.
453 If the specified list begins with a ‘^’ character, then the spec‐
454 ified key types will be placed at the head of the default set.
455 The default for this option is:
456 ecdsa-sha2-nistp256-cert-v01@openssh.com,
458 ecdsa-sha2-nistp384-cert-v01@openssh.com,
459 ecdsa-sha2-nistp521-cert-v01@openssh.com,
460 ssh-ed25519-cert-v01@openssh.com,
461 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
462 ssh-rsa-cert-v01@openssh.com,
463 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
464 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
465 The list of available key types may also be obtained using "ssh
467 -Q key".
468 HostbasedAuthentication
470 Specifies whether rhosts or /etc/hosts.equiv authentication
471 together with successful public key client host authentication is
472 allowed (host-based authentication). The default is no.
473 HostbasedUsesNameFromPacketOnly
475 Specifies whether or not the server will attempt to perform a
476 reverse name lookup when matching the name in the ~/.shosts,
477 ~/.rhosts, and /etc/hosts.equiv files during
478 HostbasedAuthentication. A setting of yes means that sshd(8)
479 uses the name supplied by the client rather than attempting to
480 resolve the name from the TCP connection itself. The default is
481 no.
482 HostCertificate
484 Specifies a file containing a public host certificate. The cer‐
485 tificate's public key must match a private host key already spec‐
486 ified by HostKey. The default behaviour of sshd(8) is not to
487 load any certificates.
488 HostKey
490 Specifies a file containing a private host key used by SSH. The
491 defaults are /etc/gsissh/ssh_host_ecdsa_key,
492 /etc/gsissh/ssh_host_ed25519_key and
493 /etc/gsissh/ssh_host_rsa_key.
494 Note that sshd(8) will refuse to use a file if it is group/world-
496 accessible and that the HostKeyAlgorithms option restricts which
497 of the keys are actually used by sshd(8).
498 It is possible to have multiple host key files. It is also pos‐
500 sible to specify public host key files instead. In this case
501 operations on the private key will be delegated to an
502 ssh-agent(1).
503 HostKeyAgent
505 Identifies the UNIX-domain socket used to communicate with an
506 agent that has access to the private host keys. If the string
507 "SSH_AUTH_SOCK" is specified, the location of the socket will be
508 read from the SSH_AUTH_SOCK environment variable.
509 HostKeyAlgorithms
511 Specifies the host key algorithms that the server offers. The
512 default is handled system-wide by crypto-policies(7). To see the
513 defaults and how to modify this default, see manual page
514 update-crypto-policies(8).
515 The list of available key types may also be obtained using "ssh
517 -Q key".
518 IgnoreRhosts
520 Specifies that .rhosts and .shosts files will not be used in
521 HostbasedAuthentication.
522 /etc/hosts.equiv and /etc/gsissh/shosts.equiv are still used.
524 The default is yes.
525 IgnoreUserKnownHosts
527 Specifies whether sshd(8) should ignore the user's
528 ~/.ssh/known_hosts during HostbasedAuthentication and use only
529 the system-wide known hosts file /etc/ssh/known_hosts. The
530 default is no.
531 IPQoS Specifies the IPv4 type-of-service or DSCP class for the connec‐
533 tion. Accepted values are af11, af12, af13, af21, af22, af23,
534 af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3, cs4, cs5,
535 cs6, cs7, ef, lowdelay, throughput, reliability, a numeric value,
536 or none to use the operating system default. This option may
537 take one or two arguments, separated by whitespace. If one argu‐
538 ment is specified, it is used as the packet class uncondition‐
539 ally. If two values are specified, the first is automatically
540 selected for interactive sessions and the second for non-interac‐
541 tive sessions. The default is af21 (Low-Latency Data) for inter‐
542 active sessions and cs1 (Lower Effort) for non-interactive ses‐
543 sions.
544 KbdInteractiveAuthentication
546 Specifies whether to allow keyboard-interactive authentication.
547 The argument to this keyword must be yes or no. The default is
548 to use whatever value ChallengeResponseAuthentication is set to
549 (by default yes).
550 KerberosAuthentication
552 Specifies whether the password provided by the user for
553 PasswordAuthentication will be validated through the Kerberos
554 KDC. To use this option, the server needs a Kerberos servtab
555 which allows the verification of the KDC's identity. The default
556 is no.
557 KerberosGetAFSToken
559 If AFS is active and the user has a Kerberos 5 TGT, attempt to
560 acquire an AFS token before accessing the user's home directory.
561 The default is no.
562 KerberosOrLocalPasswd
564 If password authentication through Kerberos fails then the pass‐
565 word will be validated via any additional local mechanism such as
566 /etc/passwd. The default is yes.
567 KerberosTicketCleanup
569 Specifies whether to automatically destroy the user's ticket
570 cache file on logout. The default is yes.
571 KerberosUniqueCCache
573 Specifies whether to store the acquired tickets in the per-ses‐
574 sion credential cache under /tmp/ or whether to use per-user cre‐
575 dential cache as configured in /etc/krb5.conf. The default value
576 no can lead to overwriting previous tickets by subseqent connec‐
577 tions to the same user account.
578 KerberosUseKuserok
580 Specifies whether to look at .k5login file for user's aliases.
581 The default is yes.
582 KexAlgorithms
584 Specifies the available KEX (Key Exchange) algorithms. Multiple
585 algorithms must be comma-separated. Alternately if the specified
586 list begins with a ‘+’ character, then the specified methods will
587 be appended to the default set instead of replacing them. If the
588 specified list begins with a ‘-’ character, then the specified
589 methods (including wildcards) will be removed from the default
590 set instead of replacing them. If the specified list begins with
591 a ‘^’ character, then the specified methods will be placed at the
592 head of the default set. The supported algorithms are:
593 curve25519-sha256
595 curve25519-sha256@libssh.org
596 diffie-hellman-group1-sha1
597 diffie-hellman-group14-sha1
598 diffie-hellman-group14-sha256
599 diffie-hellman-group16-sha512
600 diffie-hellman-group18-sha512
601 diffie-hellman-group-exchange-sha1
602 diffie-hellman-group-exchange-sha256
603 ecdh-sha2-nistp256
604 ecdh-sha2-nistp384
605 ecdh-sha2-nistp521
606 The default is handled system-wide by crypto-policies(7). To see
608 the defaults and how to modify this default, see manual page
609 update-crypto-policies(8).
610 The list of available key exchange algorithms may also be
612 obtained using "ssh -Q kex".
613 ListenAddress
615 Specifies the local addresses sshd(8) should listen on. The fol‐
616 lowing forms may be used:
617 ListenAddress hostname|address [rdomain domain]
619 ListenAddress hostname:port [rdomain domain]
620 ListenAddress IPv4_address:port [rdomain domain]
621 ListenAddress [hostname|address]:port [rdomain domain]
622 The optional rdomain qualifier requests sshd(8) listen in an
624 explicit routing domain. If port is not specified, sshd will
625 listen on the address and all Port options specified. The
626 default is to listen on all local addresses on the current
627 default routing domain. Multiple ListenAddress options are per‐
628 mitted. For more information on routing domains, see rdomain(4).
629 LoginGraceTime
631 The server disconnects after this time if the user has not suc‐
632 cessfully logged in. If the value is 0, there is no time limit.
633 The default is 120 seconds.
634 LogLevel
636 Gives the verbosity level that is used when logging messages from
637 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO,
638 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
639 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
640 higher levels of debugging output. Logging with a DEBUG level
641 violates the privacy of users and is not recommended.
642 MACs Specifies the available MAC (message authentication code) algo‐
644 rithms. The MAC algorithm is used for data integrity protection.
645 Multiple algorithms must be comma-separated. If the specified
646 list begins with a ‘+’ character, then the specified algorithms
647 will be appended to the default set instead of replacing them.
648 If the specified list begins with a ‘-’ character, then the spec‐
649 ified algorithms (including wildcards) will be removed from the
650 default set instead of replacing them. If the specified list
651 begins with a ‘^’ character, then the specified algorithms will
652 be placed at the head of the default set.
653 The algorithms that contain "-etm" calculate the MAC after
655 encryption (encrypt-then-mac). These are considered safer and
656 their use recommended. The supported MACs are:
657 hmac-md5
659 hmac-md5-96
660 hmac-sha1
661 hmac-sha1-96
662 hmac-sha2-256
663 hmac-sha2-512
664 umac-64@openssh.com
665 umac-128@openssh.com
666 hmac-md5-etm@openssh.com
667 hmac-md5-96-etm@openssh.com
668 hmac-sha1-etm@openssh.com
669 hmac-sha1-96-etm@openssh.com
670 hmac-sha2-256-etm@openssh.com
671 hmac-sha2-512-etm@openssh.com
672 umac-64-etm@openssh.com
673 umac-128-etm@openssh.com
674 The default is handled system-wide by crypto-policies(7). To see
676 the defaults and how to modify this default, see manual page
677 update-crypto-policies(8).
678 The list of available MAC algorithms may also be obtained using
680 "ssh -Q mac".
681 Match Introduces a conditional block. If all of the criteria on the
683 Match line are satisfied, the keywords on the following lines
684 override those set in the global section of the config file,
685 until either another Match line or the end of the file. If a
686 keyword appears in multiple Match blocks that are satisfied, only
687 the first instance of the keyword is applied.
688 The arguments to Match are one or more criteria-pattern pairs or
690 the single token All which matches all criteria. The available
691 criteria are User, Group, Host, LocalAddress, LocalPort, RDomain,
692 and Address (with RDomain representing the rdomain(4) on which
693 the connection was received).
694 The match patterns may consist of single entries or comma-sepa‐
696 rated lists and may use the wildcard and negation operators
697 described in the PATTERNS section of ssh_config(5).
698 The patterns in an Address criteria may additionally contain
700 addresses to match in CIDR address/masklen format, such as
701 192.0.2.0/24 or 2001:db8::/32. Note that the mask length pro‐
702 vided must be consistent with the address - it is an error to
703 specify a mask length that is too long for the address or one
704 with bits set in this host portion of the address. For example,
705 192.0.2.0/33 and 192.0.2.0/8, respectively.
706 Only a subset of keywords may be used on the lines following a
708 Match keyword. Available keywords are AcceptEnv,
709 AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding,
710 AllowTcpForwarding, AllowUsers, AuthenticationMethods,
711 AuthorizedKeysCommand, AuthorizedKeysCommandUser,
712 AuthorizedKeysFile, AuthorizedPrincipalsCommand,
713 AuthorizedPrincipalsCommandUser, AuthorizedPrincipalsFile,
714 Banner, ChrootDirectory, ClientAliveCountMax,
715 ClientAliveInterval, DenyGroups, DenyUsers, ForceCommand,
716 GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes,
717 HostbasedAuthentication, HostbasedUsesNameFromPacketOnly, IPQoS,
718 KbdInteractiveAuthentication, KerberosAuthentication,
719 KerberosUseKuserok, LogLevel, MaxAuthTries, MaxSessions,
720 PasswordAuthentication, PermitEmptyPasswords, PermitListen,
721 PermitOpen, PermitRootLogin, PermitTTY, PermitTunnel,
722 PermitUserRC, PubkeyAcceptedKeyTypes, PubkeyAuthentication,
723 RekeyLimit, RevokedKeys, RDomain, SetEnv, StreamLocalBindMask,
724 StreamLocalBindUnlink, TrustedUserCAKeys, X11DisplayOffset,
725 X11MaxDisplays, X11Forwarding and X11UseLocalhost.
726 MaxAuthTries
728 Specifies the maximum number of authentication attempts permitted
729 per connection. Once the number of failures reaches half this
730 value, additional failures are logged. The default is 6.
731 MaxSessions
733 Specifies the maximum number of open shell, login or subsystem
734 (e.g. sftp) sessions permitted per network connection. Multiple
735 sessions may be established by clients that support connection
736 multiplexing. Setting MaxSessions to 1 will effectively disable
737 session multiplexing, whereas setting it to 0 will prevent all
738 shell, login and subsystem sessions while still permitting for‐
739 warding. The default is 10.
740 MaxStartups
742 Specifies the maximum number of concurrent unauthenticated con‐
743 nections to the SSH daemon. Additional connections will be
744 dropped until authentication succeeds or the LoginGraceTime
745 expires for a connection. The default is 10:30:100.
746 Alternatively, random early drop can be enabled by specifying the
748 three colon separated values start:rate:full (e.g. "10:30:60").
749 sshd(8) will refuse connection attempts with a probability of
750 rate/100 (30%) if there are currently start (10) unauthenticated
751 connections. The probability increases linearly and all connec‐
752 tion attempts are refused if the number of unauthenticated con‐
753 nections reaches full (60).
754 PasswordAuthentication
756 Specifies whether password authentication is allowed. The
757 default is yes.
758 PermitEmptyPasswords
760 When password authentication is allowed, it specifies whether the
761 server allows login to accounts with empty password strings. The
762 default is no.
763 PermitListen
765 Specifies the addresses/ports on which a remote TCP port forward‐
766 ing may listen. The listen specification must be one of the fol‐
767 lowing forms:
768 PermitListen port
770 PermitListen host:port
771 Multiple permissions may be specified by separating them with
773 whitespace. An argument of any can be used to remove all
774 restrictions and permit any listen requests. An argument of none
775 can be used to prohibit all listen requests. The host name may
776 contain wildcards as described in the PATTERNS section in
777 ssh_config(5). The wildcard ‘*’ can also be used in place of a
778 port number to allow all ports. By default all port forwarding
779 listen requests are permitted. Note that the GatewayPorts option
780 may further restrict which addresses may be listened on. Note
781 also that ssh(1) will request a listen host of “localhost” if no
782 listen host was specifically requested, and this name is treated
783 differently to explicit localhost addresses of “127.0.0.1” and
784 “::1”.
785 PermitOpen
787 Specifies the destinations to which TCP port forwarding is per‐
788 mitted. The forwarding specification must be one of the follow‐
789 ing forms:
790 PermitOpen host:port
792 PermitOpen IPv4_addr:port
793 PermitOpen [IPv6_addr]:port
794 Multiple forwards may be specified by separating them with white‐
796 space. An argument of any can be used to remove all restrictions
797 and permit any forwarding requests. An argument of none can be
798 used to prohibit all forwarding requests. The wildcard ‘*’ can
799 be used for host or port to allow all hosts or ports, respec‐
800 tively. By default all port forwarding requests are permitted.
801 PermitRootLogin
803 Specifies whether root can log in using ssh(1). The argument
804 must be yes, prohibit-password, forced-commands-only, or no. The
805 default is prohibit-password.
806 If this option is set to prohibit-password (or its deprecated
808 alias, without-password), password and keyboard-interactive
809 authentication are disabled for root.
810 If this option is set to forced-commands-only, root login with
812 public key authentication will be allowed, but only if the
813 command option has been specified (which may be useful for taking
814 remote backups even if root login is normally not allowed). All
815 other authentication methods are disabled for root.
816 If this option is set to no, root is not allowed to log in.
818 PermitTTY
820 Specifies whether pty(4) allocation is permitted. The default is
821 yes.
822 PermitTunnel
824 Specifies whether tun(4) device forwarding is allowed. The argu‐
825 ment must be yes, point-to-point (layer 3), ethernet (layer 2),
826 or no. Specifying yes permits both point-to-point and ethernet.
827 The default is no.
828 Independent of this setting, the permissions of the selected
830 tun(4) device must allow access to the user.
831 PermitUserEnvironment
833 Specifies whether ~/.ssh/environment and environment= options in
834 ~/.ssh/authorized_keys are processed by sshd(8). Valid options
835 are yes, no or a pattern-list specifying which environment vari‐
836 able names to accept (for example "LANG,LC_*"). The default is
837 no. Enabling environment processing may enable users to bypass
838 access restrictions in some configurations using mechanisms such
839 as LD_PRELOAD.
840 PermitUserRC
842 Specifies whether any ~/.ssh/rc file is executed. The default is
843 yes.
844 PidFile
846 Specifies the file that contains the process ID of the SSH dae‐
847 mon, or none to not write one. The default is
848 /var/run/gsisshd.pid.
849 Port Specifies the port number that sshd(8) listens on. The default
851 is 22. Multiple options of this type are permitted. See also
852 ListenAddress.
853 PrintLastLog
855 Specifies whether sshd(8) should print the date and time of the
856 last user login when a user logs in interactively. The default
857 is yes.
858 PrintMotd
860 Specifies whether sshd(8) should print /etc/motd when a user logs
861 in interactively. (On some systems it is also printed by the
862 shell, /etc/profile, or equivalent.) The default is yes.
863 PubkeyAcceptedKeyTypes
865 Specifies the key types that will be accepted for public key
866 authentication as a list of comma-separated patterns. Alter‐
867 nately if the specified list begins with a ‘+’ character, then
868 the specified key types will be appended to the default set
869 instead of replacing them. If the specified list begins with a
870 ‘-’ character, then the specified key types (including wildcards)
871 will be removed from the default set instead of replacing them.
872 If the specified list begins with a ‘^’ character, then the spec‐
873 ified key types will be placed at the head of the default set.
874 The default is handled system-wide by crypto-policies(7). To see
875 the defaults and how to modify this default, see manual page
876 update-crypto-policies(8).
877 The list of available key types may also be obtained using "ssh
879 -Q key".
880 PubkeyAuthentication
882 Specifies whether public key authentication is allowed. The
883 default is yes.
884 RekeyLimit
886 Specifies the maximum amount of data that may be transmitted
887 before the session key is renegotiated, optionally followed a
888 maximum amount of time that may pass before the session key is
889 renegotiated. The first argument is specified in bytes and may
890 have a suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes,
891 Megabytes, or Gigabytes, respectively. The default is between
892 ‘1G’ and ‘4G’, depending on the cipher. The optional second
893 value is specified in seconds and may use any of the units docu‐
894 mented in the TIME FORMATS section. The default value for
895 RekeyLimit is default none, which means that rekeying is per‐
896 formed after the cipher's default amount of data has been sent or
897 received and no time based rekeying is done.
898 RevokedKeys
900 Specifies revoked public keys file, or none to not use one. Keys
901 listed in this file will be refused for public key authentica‐
902 tion. Note that if this file is not readable, then public key
903 authentication will be refused for all users. Keys may be speci‐
904 fied as a text file, listing one public key per line, or as an
905 OpenSSH Key Revocation List (KRL) as generated by ssh-keygen(1).
906 For more information on KRLs, see the KEY REVOCATION LISTS sec‐
907 tion in ssh-keygen(1).
908 RDomain
910 Specifies an explicit routing domain that is applied after
911 authentication has completed. The user session, as well and any
912 forwarded or listening IP sockets, will be bound to this
913 rdomain(4). If the routing domain is set to %D, then the domain
914 in which the incoming connection was received will be applied.
915 SetEnv Specifies one or more environment variables to set in child ses‐
917 sions started by sshd(8) as “NAME=VALUE”. The environment value
918 may be quoted (e.g. if it contains whitespace characters). Envi‐
919 ronment variables set by SetEnv override the default environment
920 and any variables specified by the user via AcceptEnv or
921 PermitUserEnvironment.
922 StreamLocalBindMask
924 Sets the octal file creation mode mask (umask) used when creating
925 a Unix-domain socket file for local or remote port forwarding.
926 This option is only used for port forwarding to a Unix-domain
927 socket file.
928 The default value is 0177, which creates a Unix-domain socket
930 file that is readable and writable only by the owner. Note that
931 not all operating systems honor the file mode on Unix-domain
932 socket files.
933 StreamLocalBindUnlink
935 Specifies whether to remove an existing Unix-domain socket file
936 for local or remote port forwarding before creating a new one.
937 If the socket file already exists and StreamLocalBindUnlink is
938 not enabled, sshd will be unable to forward the port to the Unix-
939 domain socket file. This option is only used for port forwarding
940 to a Unix-domain socket file.
941 The argument must be yes or no. The default is no.
943 StrictModes
945 Specifies whether sshd(8) should check file modes and ownership
946 of the user's files and home directory before accepting login.
947 This is normally desirable because novices sometimes accidentally
948 leave their directory or files world-writable. The default is
949 yes. Note that this does not apply to ChrootDirectory, whose
950 permissions and ownership are checked unconditionally.
951 Subsystem
953 Configures an external subsystem (e.g. file transfer daemon).
954 Arguments should be a subsystem name and a command (with optional
955 arguments) to execute upon subsystem request.
956 The command sftp-server implements the SFTP file transfer subsys‐
958 tem.
959 Alternately the name internal-sftp implements an in-process SFTP
961 server. This may simplify configurations using ChrootDirectory
962 to force a different filesystem root on clients.
963 By default no subsystems are defined.
965 SyslogFacility
967 Gives the facility code that is used when logging messages from
968 sshd(8). The possible values are: DAEMON, USER, AUTH, AUTHPRIV,
969 LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
970 The default is AUTH.
971 TCPKeepAlive
973 Specifies whether the system should send TCP keepalive messages
974 to the other side. If they are sent, death of the connection or
975 crash of one of the machines will be properly noticed. However,
976 this means that connections will die if the route is down tempo‐
977 rarily, and some people find it annoying. On the other hand, if
978 TCP keepalives are not sent, sessions may hang indefinitely on
979 the server, leaving "ghost" users and consuming server resources.
980 The default is yes (to send TCP keepalive messages), and the
982 server will notice if the network goes down or the client host
983 crashes. This avoids infinitely hanging sessions.
984 To disable TCP keepalive messages, the value should be set to no.
986 TrustedUserCAKeys
988 Specifies a file containing public keys of certificate authori‐
989 ties that are trusted to sign user certificates for authentica‐
990 tion, or none to not use one. Keys are listed one per line;
991 empty lines and comments starting with ‘#’ are allowed. If a
992 certificate is presented for authentication and has its signing
993 CA key listed in this file, then it may be used for authentica‐
994 tion for any user listed in the certificate's principals list.
995 Note that certificates that lack a list of principals will not be
996 permitted for authentication using TrustedUserCAKeys. For more
997 details on certificates, see the CERTIFICATES section in
998 ssh-keygen(1).
999 UseDNS Specifies whether sshd(8) should look up the remote host name,
1001 and to check that the resolved host name for the remote IP
1002 address maps back to the very same IP address.
1003 If this option is set to no (the default) then only addresses and
1005 not host names may be used in ~/.ssh/authorized_keys from and
1006 sshd_config Match Host directives.
1007 UsePAM Enables the Pluggable Authentication Module interface. If set to
1009 yes this will enable PAM authentication using
1010 ChallengeResponseAuthentication and PasswordAuthentication in
1011 addition to PAM account and session module processing for all
1012 authentication types.
1013 Because PAM challenge-response authentication usually serves an
1015 equivalent role to password authentication, you should disable
1016 either PasswordAuthentication or ChallengeResponseAuthentication.
1017 If UsePAM is enabled, you will not be able to run sshd(8) as a
1019 non-root user. The default is no.
1020 PermitPAMUserChange
1022 If set to yes this will enable PAM authentication to change the
1023 name of the user being authenticated. The default is no.
1024 VersionAddendum
1026 Optionally specifies additional text to append to the SSH proto‐
1027 col banner sent by the server upon connection. The default is
1028 none.
1029 X11DisplayOffset
1031 Specifies the first display number available for sshd(8)'s X11
1032 forwarding. This prevents sshd from interfering with real X11
1033 servers. The default is 10.
1034 X11MaxDisplays
1036 Specifies the maximum number of displays available for sshd(8)'s
1037 X11 forwarding. This prevents sshd from exhausting local ports.
1038 The default is 1000.
1039 X11Forwarding
1041 Specifies whether X11 forwarding is permitted. The argument must
1042 be yes or no. The default is no.
1043 When X11 forwarding is enabled, there may be additional exposure
1045 to the server and to client displays if the sshd(8) proxy display
1046 is configured to listen on the wildcard address (see
1047 X11UseLocalhost), though this is not the default. Additionally,
1048 the authentication spoofing and authentication data verification
1049 and substitution occur on the client side. The security risk of
1050 using X11 forwarding is that the client's X11 display server may
1051 be exposed to attack when the SSH client requests forwarding (see
1052 the warnings for ForwardX11 in ssh_config(5)). A system adminis‐
1053 trator may have a stance in which they want to protect clients
1054 that may expose themselves to attack by unwittingly requesting
1055 X11 forwarding, which can warrant a no setting.
1056 Note that disabling X11 forwarding does not prevent users from
1058 forwarding X11 traffic, as users can always install their own
1059 forwarders.
1060 X11UseLocalhost
1062 Specifies whether sshd(8) should bind the X11 forwarding server
1063 to the loopback address or to the wildcard address. By default,
1064 sshd binds the forwarding server to the loopback address and sets
1065 the hostname part of the DISPLAY environment variable to
1066 localhost. This prevents remote hosts from connecting to the
1067 proxy display. However, some older X11 clients may not function
1068 with this configuration. X11UseLocalhost may be set to no to
1069 specify that the forwarding server should be bound to the wild‐
1070 card address. The argument must be yes or no. The default is
1071 yes.
1072 XAuthLocation
1074 Specifies the full pathname of the xauth(1) program, or none to
1075 not use one. The default is /usr/bin/xauth.
1076
1078 sshd(8) command-line arguments and configuration file options that spec‐
1079 ify time may be expressed using a sequence of the form: time[qualifier],
1080 where time is a positive integer value and qualifier is one of the fol‐
1081 lowing:
1082 ⟨none⟩ seconds
1084 s | S seconds
1085 m | M minutes
1086 h | H hours
1087 d | D days
1088 w | W weeks
1089 Each member of the sequence is added together to calculate the total time
1091 value.
1092 Time format examples:
1094 600 600 seconds (10 minutes)
1096 10m 10 minutes
1097 1h30m 1 hour 30 minutes (90 minutes)
1098
1100 Arguments to some keywords can make use of tokens, which are expanded at
1101 runtime:
1102 %% A literal ‘%’.
1104 %D The routing domain in which the incoming connection was
1105 received.
1106 %F The fingerprint of the CA key.
1107 %f The fingerprint of the key or certificate.
1108 %h The home directory of the user.
1109 %i The key ID in the certificate.
1110 %K The base64-encoded CA key.
1111 %k The base64-encoded key or certificate for authentication.
1112 %s The serial number of the certificate.
1113 %T The type of the CA key.
1114 %t The key or certificate type.
1115 %U The numeric user ID of the target user.
1116 %u The username.
1117 AuthorizedKeysCommand accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1119 AuthorizedKeysFile accepts the tokens %%, %h, %U, and %u.
1121 AuthorizedPrincipalsCommand accepts the tokens %%, %F, %f, %h, %i, %K,
1123 %k, %s, %T, %t, %U, and %u.
1124 AuthorizedPrincipalsFile accepts the tokens %%, %h, %U, and %u.
1126 ChrootDirectory accepts the tokens %%, %h, %U, and %u.
1128 RoutingDomain accepts the token %D.
1130
1132 /etc/gsissh/sshd_config
1133 Contains configuration data for sshd(8). This file should be
1134 writable by root only, but it is recommended (though not neces‐
1135 sary) that it be world-readable.
1136
1138 sftp-server(8), sshd(8)
1139
1141 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
1142 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
1143 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
1144 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
1145 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
1146 for privilege separation.
1147BSD December 31, 2019 BSD