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 value begins with a ‘+’ character,
253 then the specified ciphers will be appended to the default set
254 instead of replacing them. If the specified value begins with a
255 ‘-’ character, then the specified ciphers (including wildcards)
256 will be removed from the default set instead of replacing them.
257 The supported ciphers are:
259 3des-cbc
261 aes128-cbc
262 aes192-cbc
263 aes256-cbc
264 aes128-ctr
265 aes192-ctr
266 aes256-ctr
267 aes128-gcm@openssh.com
268 aes256-gcm@openssh.com
269 chacha20-poly1305@openssh.com
270 The default is handled system-wide by crypto-policies(7). To see
272 the defaults and how to modifuy this default, see manual page
273 update-crypto-policies(8).
274 The list of available ciphers may also be obtained using "ssh -Q
276 cipher".
277 ClientAliveCountMax
279 Sets the number of client alive messages which may be sent with‐
280 out sshd(8) receiving any messages back from the client. If this
281 threshold is reached while client alive messages are being sent,
282 sshd will disconnect the client, terminating the session. It is
283 important to note that the use of client alive messages is very
284 different from TCPKeepAlive. The client alive messages are sent
285 through the encrypted channel and therefore will not be spoofa‐
286 ble. The TCP keepalive option enabled by TCPKeepAlive is spoofa‐
287 ble. The client alive mechanism is valuable when the client or
288 server depend on knowing when a connection has become inactive.
289 The default value is 3. If ClientAliveInterval is set to 15, and
291 ClientAliveCountMax is left at the default, unresponsive SSH
292 clients will be disconnected after approximately 45 seconds.
293 ClientAliveInterval
295 Sets a timeout interval in seconds after which if no data has
296 been received from the client, sshd(8) will send a message
297 through the encrypted channel to request a response from the
298 client. The default is 0, indicating that these messages will
299 not be sent to the client.
300 Compression
302 Specifies whether compression is enabled after the user has
303 authenticated successfully. The argument must be yes, delayed (a
304 legacy synonym for yes) or no. The default is yes.
305 DenyGroups
307 This keyword can be followed by a list of group name patterns,
308 separated by spaces. Login is disallowed for users whose primary
309 group or supplementary group list matches one of the patterns.
310 Only group names are valid; a numerical group ID is not recog‐
311 nized. By default, login is allowed for all groups. The
312 allow/deny directives are processed in the following order:
313 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
314 See PATTERNS in ssh_config(5) for more information on patterns.
316 DenyUsers
318 This keyword can be followed by a list of user name patterns,
319 separated by spaces. Login is disallowed for user names that
320 match one of the patterns. Only user names are valid; a numeri‐
321 cal user ID is not recognized. By default, login is allowed for
322 all users. If the pattern takes the form USER@HOST then USER and
323 HOST are separately checked, restricting logins to particular
324 users from particular hosts. HOST criteria may additionally con‐
325 tain addresses to match in CIDR address/masklen format. The
326 allow/deny directives are processed in the following order:
327 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
328 See PATTERNS in ssh_config(5) for more information on patterns.
330 DisableForwarding
332 Disables all forwarding features, including X11, ssh-agent(1),
333 TCP and StreamLocal. This option overrides all other forwarding-
334 related options and may simplify restricted configurations.
335 ExposeAuthInfo
337 Writes a temporary file containing a list of authentication meth‐
338 ods and public credentials (e.g. keys) used to authenticate the
339 user. The location of the file is exposed to the user session
340 through the SSH_USER_AUTH environment variable. The default is
341 no.
342 FingerprintHash
344 Specifies the hash algorithm used when logging key fingerprints.
345 Valid options are: md5 and sha256. The default is sha256.
346 ForceCommand
348 Forces the execution of the command specified by ForceCommand,
349 ignoring any command supplied by the client and ~/.ssh/rc if
350 present. The command is invoked by using the user's login shell
351 with the -c option. This applies to shell, command, or subsystem
352 execution. It is most useful inside a Match block. The command
353 originally supplied by the client is available in the
354 SSH_ORIGINAL_COMMAND environment variable. Specifying a command
355 of internal-sftp will force the use of an in-process SFTP server
356 that requires no support files when used with ChrootDirectory.
357 The default is none.
358 GatewayPorts
360 Specifies whether remote hosts are allowed to connect to ports
361 forwarded for the client. By default, sshd(8) binds remote port
362 forwardings to the loopback address. This prevents other remote
363 hosts from connecting to forwarded ports. GatewayPorts can be
364 used to specify that sshd should allow remote port forwardings to
365 bind to non-loopback addresses, thus allowing other hosts to con‐
366 nect. The argument may be no to force remote port forwardings to
367 be available to the local host only, yes to force remote port
368 forwardings to bind to the wildcard address, or clientspecified
369 to allow the client to select the address to which the forwarding
370 is bound. The default is no.
371 GSIAllowLimitedProxy
373 Specifies whether to accept limited proxy credentials for authen‐
374 tication. The default is no.
375 GSSAPIAuthentication
377 Specifies whether user authentication based on GSSAPI is allowed.
378 The default is yes.
379 GSSAPICleanupCredentials
381 Specifies whether to automatically destroy the user's credentials
382 cache on logout. The default is yes.
383 GSSAPICredentialsPath
385 If specified, the delegated GSSAPI credential is stored in the
386 given path, overwriting any existing credentials. Paths can be
387 specified with syntax similar to the AuthorizedKeysFile option
388 (i.e., accepting %h and %u tokens). When using this option, set‐
389 ting 'GssapiCleanupCredentials no' is recommended, so logging out
390 of one session doesn't remove the credentials in use by another
391 session of the same user. Currently only implemented for the GSI
392 mechanism.
393 GSSAPIDelegateCredentials
395 Specifies whether delegated credentials are stored in the user's
396 environment. The default is yes.
397 GSSAPIEnablek5users
399 Specifies whether to look at .k5users file for GSSAPI authentica‐
400 tion access control. Further details are described in ksu(1).
401 The default is no.
402 GSSAPIKeyExchange
404 Specifies whether key exchange based on GSSAPI is allowed. GSSAPI
405 key exchange doesn't rely on ssh keys to verify host identity.
406 The default is yes.
407 GSSAPIStrictAcceptorCheck
409 Determines whether to be strict about the identity of the GSSAPI
410 acceptor a client authenticates against. If set to yes then the
411 client must authenticate against the host service on the current
412 hostname. If set to no then the client may authenticate against
413 any service key stored in the machine's default store. This
414 facility is provided to assist with operation on multi homed
415 machines. The default is yes.
416 GSSAPIStoreCredentialsOnRekey
418 Controls whether the user's GSSAPI credentials should be updated
419 following a successful connection rekeying. This option can be
420 used to accepted renewed or updated credentials from a compatible
421 client. The default is “no”.
422 For this to work GSSAPIKeyExchange needs to be enabled in the
424 server and also used by the client.
425 GSSAPIKexAlgorithms
427 The list of key exchange algorithms that are accepted by GSSAPI
428 key exchange. Possible values are
429 gss-gex-sha1-,
431 gss-group1-sha1-,
432 gss-group14-sha1-,
433 gss-group14-sha256-,
434 gss-group16-sha512-,
435 gss-nistp256-sha256-,
436 gss-curve25519-sha256-
437 The default is handled system-wide by crypto-policies(7). To see
439 the defaults and how to modifuy this default, see manual page
440 update-crypto-policies(8).
441 HostbasedAcceptedKeyTypes
443 Specifies the key types that will be accepted for hostbased
444 authentication as a list of comma-separated patterns. Alter‐
445 nately if the specified value begins with a ‘+’ character, then
446 the specified key types will be appended to the default set
447 instead of replacing them. If the specified value begins with a
448 ‘-’ character, then the specified key types (including wildcards)
449 will be removed from the default set instead of replacing them.
450 The default for this option is:
451 ecdsa-sha2-nistp256-cert-v01@openssh.com,
453 ecdsa-sha2-nistp384-cert-v01@openssh.com,
454 ecdsa-sha2-nistp521-cert-v01@openssh.com,
455 ssh-ed25519-cert-v01@openssh.com,
456 rsa-sha2-512-cert-v01@openssh.com,rsa-sha2-256-cert-v01@openssh.com,
457 ssh-rsa-cert-v01@openssh.com,
458 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
459 ssh-ed25519,rsa-sha2-512,rsa-sha2-256,ssh-rsa
460 The list of available key types may also be obtained using "ssh
462 -Q key".
463 HostbasedAuthentication
465 Specifies whether rhosts or /etc/hosts.equiv authentication
466 together with successful public key client host authentication is
467 allowed (host-based authentication). The default is no.
468 HostbasedUsesNameFromPacketOnly
470 Specifies whether or not the server will attempt to perform a
471 reverse name lookup when matching the name in the ~/.shosts,
472 ~/.rhosts, and /etc/hosts.equiv files during
473 HostbasedAuthentication. A setting of yes means that sshd(8)
474 uses the name supplied by the client rather than attempting to
475 resolve the name from the TCP connection itself. The default is
476 no.
477 HostCertificate
479 Specifies a file containing a public host certificate. The cer‐
480 tificate's public key must match a private host key already spec‐
481 ified by HostKey. The default behaviour of sshd(8) is not to
482 load any certificates.
483 HostKey
485 Specifies a file containing a private host key used by SSH. The
486 defaults are /etc/gsissh/ssh_host_ecdsa_key,
487 /etc/gsissh/ssh_host_ed25519_key and
488 /etc/gsissh/ssh_host_rsa_key.
489 Note that sshd(8) will refuse to use a file if it is group/world-
491 accessible and that the HostKeyAlgorithms option restricts which
492 of the keys are actually used by sshd(8).
493 It is possible to have multiple host key files. It is also pos‐
495 sible to specify public host key files instead. In this case
496 operations on the private key will be delegated to an
497 ssh-agent(1).
498 HostKeyAgent
500 Identifies the UNIX-domain socket used to communicate with an
501 agent that has access to the private host keys. If the string
502 "SSH_AUTH_SOCK" is specified, the location of the socket will be
503 read from the SSH_AUTH_SOCK environment variable.
504 HostKeyAlgorithms
506 Specifies the host key algorithms that the server offers. The
507 default is handled system-wide by crypto-policies(7). To see the
508 defaults and how to modifuy this default, see manual page
509 update-crypto-policies(8).
510 The list of available key types may also be obtained using "ssh
512 -Q key".
513 IgnoreRhosts
515 Specifies that .rhosts and .shosts files will not be used in
516 HostbasedAuthentication.
517 /etc/hosts.equiv and /etc/gsissh/shosts.equiv are still used.
519 The default is yes.
520 IgnoreUserKnownHosts
522 Specifies whether sshd(8) should ignore the user's
523 ~/.ssh/known_hosts during HostbasedAuthentication and use only
524 the system-wide known hosts file /etc/ssh/known_hosts. The
525 default is no.
526 IPQoS Specifies the IPv4 type-of-service or DSCP class for the connec‐
528 tion. Accepted values are af11, af12, af13, af21, af22, af23,
529 af31, af32, af33, af41, af42, af43, cs0, cs1, cs2, cs3, cs4, cs5,
530 cs6, cs7, ef, lowdelay, throughput, reliability, a numeric value,
531 or none to use the operating system default. This option may
532 take one or two arguments, separated by whitespace. If one argu‐
533 ment is specified, it is used as the packet class uncondition‐
534 ally. If two values are specified, the first is automatically
535 selected for interactive sessions and the second for non-interac‐
536 tive sessions. The default is af21 (Low-Latency Data) for inter‐
537 active sessions and cs1 (Lower Effort) for non-interactive ses‐
538 sions.
539 KbdInteractiveAuthentication
541 Specifies whether to allow keyboard-interactive authentication.
542 The argument to this keyword must be yes or no. The default is
543 to use whatever value ChallengeResponseAuthentication is set to
544 (by default yes).
545 KerberosAuthentication
547 Specifies whether the password provided by the user for
548 PasswordAuthentication will be validated through the Kerberos
549 KDC. To use this option, the server needs a Kerberos servtab
550 which allows the verification of the KDC's identity. The default
551 is no.
552 KerberosGetAFSToken
554 If AFS is active and the user has a Kerberos 5 TGT, attempt to
555 acquire an AFS token before accessing the user's home directory.
556 The default is no.
557 KerberosOrLocalPasswd
559 If password authentication through Kerberos fails then the pass‐
560 word will be validated via any additional local mechanism such as
561 /etc/passwd. The default is yes.
562 KerberosTicketCleanup
564 Specifies whether to automatically destroy the user's ticket
565 cache file on logout. The default is yes.
566 KerberosUniqueCCache
568 Specifies whether to store the acquired tickets in the per-ses‐
569 sion credential cache under /tmp/ or whether to use per-user cre‐
570 dential cache as configured in /etc/krb5.conf. The default value
571 no can lead to overwriting previous tickets by subseqent connec‐
572 tions to the same user account.
573 KerberosUseKuserok
575 Specifies whether to look at .k5login file for user's aliases.
576 The default is yes.
577 KexAlgorithms
579 Specifies the available KEX (Key Exchange) algorithms. Multiple
580 algorithms must be comma-separated. Alternately if the specified
581 value begins with a ‘+’ character, then the specified methods
582 will be appended to the default set instead of replacing them.
583 If the specified value begins with a ‘-’ character, then the
584 specified methods (including wildcards) will be removed from the
585 default set instead of replacing them. The supported algorithms
586 are:
587 curve25519-sha256
589 curve25519-sha256@libssh.org
590 diffie-hellman-group1-sha1
591 diffie-hellman-group14-sha1
592 diffie-hellman-group14-sha256
593 diffie-hellman-group16-sha512
594 diffie-hellman-group18-sha512
595 diffie-hellman-group-exchange-sha1
596 diffie-hellman-group-exchange-sha256
597 ecdh-sha2-nistp256
598 ecdh-sha2-nistp384
599 ecdh-sha2-nistp521
600 The default is handled system-wide by crypto-policies(7). To see
602 the defaults and how to modifuy this default, see manual page
603 update-crypto-policies(8).
604 The list of available key exchange algorithms may also be
606 obtained using "ssh -Q kex".
607 ListenAddress
609 Specifies the local addresses sshd(8) should listen on. The fol‐
610 lowing forms may be used:
611 ListenAddress hostname|address [rdomain domain]
613 ListenAddress hostname:port [rdomain domain]
614 ListenAddress IPv4_address:port [rdomain domain]
615 ListenAddress [hostname|address]:port [rdomain domain]
616 The optional rdomain qualifier requests sshd(8) listen in an
618 explicit routing domain. If port is not specified, sshd will
619 listen on the address and all Port options specified. The
620 default is to listen on all local addresses on the current
621 default routing domain. Multiple ListenAddress options are per‐
622 mitted. For more information on routing domains, see rdomain(4).
623 LoginGraceTime
625 The server disconnects after this time if the user has not suc‐
626 cessfully logged in. If the value is 0, there is no time limit.
627 The default is 120 seconds.
628 LogLevel
630 Gives the verbosity level that is used when logging messages from
631 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO,
632 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
633 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
634 higher levels of debugging output. Logging with a DEBUG level
635 violates the privacy of users and is not recommended.
636 MACs Specifies the available MAC (message authentication code) algo‐
638 rithms. The MAC algorithm is used for data integrity protection.
639 Multiple algorithms must be comma-separated. If the specified
640 value begins with a ‘+’ character, then the specified algorithms
641 will be appended to the default set instead of replacing them.
642 If the specified value begins with a ‘-’ character, then the
643 specified algorithms (including wildcards) will be removed from
644 the default set instead of replacing them.
645 The algorithms that contain "-etm" calculate the MAC after
647 encryption (encrypt-then-mac). These are considered safer and
648 their use recommended. The supported MACs are:
649 hmac-md5
651 hmac-md5-96
652 hmac-sha1
653 hmac-sha1-96
654 hmac-sha2-256
655 hmac-sha2-512
656 umac-64@openssh.com
657 umac-128@openssh.com
658 hmac-md5-etm@openssh.com
659 hmac-md5-96-etm@openssh.com
660 hmac-sha1-etm@openssh.com
661 hmac-sha1-96-etm@openssh.com
662 hmac-sha2-256-etm@openssh.com
663 hmac-sha2-512-etm@openssh.com
664 umac-64-etm@openssh.com
665 umac-128-etm@openssh.com
666 The default is handled system-wide by crypto-policies(7). To see
668 the defaults and how to modifuy this default, see manual page
669 update-crypto-policies(8).
670 The list of available MAC algorithms may also be obtained using
672 "ssh -Q mac".
673 Match Introduces a conditional block. If all of the criteria on the
675 Match line are satisfied, the keywords on the following lines
676 override those set in the global section of the config file,
677 until either another Match line or the end of the file. If a
678 keyword appears in multiple Match blocks that are satisfied, only
679 the first instance of the keyword is applied.
680 The arguments to Match are one or more criteria-pattern pairs or
682 the single token All which matches all criteria. The available
683 criteria are User, Group, Host, LocalAddress, LocalPort, RDomain,
684 and Address (with RDomain representing the rdomain(4) on which
685 the connection was received).
686 The match patterns may consist of single entries or comma-sepa‐
688 rated lists and may use the wildcard and negation operators
689 described in the PATTERNS section of ssh_config(5).
690 The patterns in an Address criteria may additionally contain
692 addresses to match in CIDR address/masklen format, such as
693 192.0.2.0/24 or 2001:db8::/32. Note that the mask length pro‐
694 vided must be consistent with the address - it is an error to
695 specify a mask length that is too long for the address or one
696 with bits set in this host portion of the address. For example,
697 192.0.2.0/33 and 192.0.2.0/8, respectively.
698 Only a subset of keywords may be used on the lines following a
700 Match keyword. Available keywords are AcceptEnv,
701 AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding,
702 AllowTcpForwarding, AllowUsers, AuthenticationMethods,
703 AuthorizedKeysCommand, AuthorizedKeysCommandUser,
704 AuthorizedKeysFile, AuthorizedPrincipalsCommand,
705 AuthorizedPrincipalsCommandUser, AuthorizedPrincipalsFile,
706 Banner, ChrootDirectory, ClientAliveCountMax,
707 ClientAliveInterval, DenyGroups, DenyUsers, ForceCommand,
708 GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes,
709 HostbasedAuthentication, HostbasedUsesNameFromPacketOnly, IPQoS,
710 KbdInteractiveAuthentication, KerberosAuthentication,
711 KerberosUseKuserok, LogLevel, MaxAuthTries, MaxSessions,
712 PasswordAuthentication, PermitEmptyPasswords, PermitListen,
713 PermitOpen, PermitRootLogin, PermitTTY, PermitTunnel,
714 PermitUserRC, PubkeyAcceptedKeyTypes, PubkeyAuthentication,
715 RekeyLimit, RevokedKeys, RDomain, SetEnv, StreamLocalBindMask,
716 StreamLocalBindUnlink, TrustedUserCAKeys, X11DisplayOffset,
717 X11MaxDisplays, X11Forwarding and X11UseLocalHost.
718 MaxAuthTries
720 Specifies the maximum number of authentication attempts permitted
721 per connection. Once the number of failures reaches half this
722 value, additional failures are logged. The default is 6.
723 MaxSessions
725 Specifies the maximum number of open shell, login or subsystem
726 (e.g. sftp) sessions permitted per network connection. Multiple
727 sessions may be established by clients that support connection
728 multiplexing. Setting MaxSessions to 1 will effectively disable
729 session multiplexing, whereas setting it to 0 will prevent all
730 shell, login and subsystem sessions while still permitting for‐
731 warding. The default is 10.
732 MaxStartups
734 Specifies the maximum number of concurrent unauthenticated con‐
735 nections to the SSH daemon. Additional connections will be
736 dropped until authentication succeeds or the LoginGraceTime
737 expires for a connection. The default is 10:30:100.
738 Alternatively, random early drop can be enabled by specifying the
740 three colon separated values start:rate:full (e.g. "10:30:60").
741 sshd(8) will refuse connection attempts with a probability of
742 rate/100 (30%) if there are currently start (10) unauthenticated
743 connections. The probability increases linearly and all connec‐
744 tion attempts are refused if the number of unauthenticated con‐
745 nections reaches full (60).
746 PasswordAuthentication
748 Specifies whether password authentication is allowed. The
749 default is yes.
750 PermitEmptyPasswords
752 When password authentication is allowed, it specifies whether the
753 server allows login to accounts with empty password strings. The
754 default is no.
755 PermitListen
757 Specifies the addresses/ports on which a remote TCP port forward‐
758 ing may listen. The listen specification must be one of the fol‐
759 lowing forms:
760 PermitListen port
762 PermitListen host:port
763 Multiple permissions may be specified by separating them with
765 whitespace. An argument of any can be used to remove all
766 restrictions and permit any listen requests. An argument of none
767 can be used to prohibit all listen requests. The host name may
768 contain wildcards as described in the PATTERNS section in
769 ssh_config(5). The wildcard ‘*’ can also be used in place of a
770 port number to allow all ports. By default all port forwarding
771 listen requests are permitted. Note that the GatewayPorts option
772 may further restrict which addresses may be listened on. Note
773 also that ssh(1) will request a listen host of “localhost” if no
774 listen host was specifically requested, and this name is treated
775 differently to explicit localhost addresses of “127.0.0.1” and
776 “::1”.
777 PermitOpen
779 Specifies the destinations to which TCP port forwarding is per‐
780 mitted. The forwarding specification must be one of the follow‐
781 ing forms:
782 PermitOpen host:port
784 PermitOpen IPv4_addr:port
785 PermitOpen [IPv6_addr]:port
786 Multiple forwards may be specified by separating them with white‐
788 space. An argument of any can be used to remove all restrictions
789 and permit any forwarding requests. An argument of none can be
790 used to prohibit all forwarding requests. The wildcard ‘*’ can
791 be used for host or port to allow all hosts or ports, respec‐
792 tively. By default all port forwarding requests are permitted.
793 PermitRootLogin
795 Specifies whether root can log in using ssh(1). The argument
796 must be yes, prohibit-password, forced-commands-only, or no. The
797 default is prohibit-password.
798 If this option is set to prohibit-password (or its deprecated
800 alias, without-password), password and keyboard-interactive
801 authentication are disabled for root.
802 If this option is set to forced-commands-only, root login with
804 public key authentication will be allowed, but only if the
805 command option has been specified (which may be useful for taking
806 remote backups even if root login is normally not allowed). All
807 other authentication methods are disabled for root.
808 If this option is set to no, root is not allowed to log in.
810 PermitTTY
812 Specifies whether pty(4) allocation is permitted. The default is
813 yes.
814 PermitTunnel
816 Specifies whether tun(4) device forwarding is allowed. The argu‐
817 ment must be yes, point-to-point (layer 3), ethernet (layer 2),
818 or no. Specifying yes permits both point-to-point and ethernet.
819 The default is no.
820 Independent of this setting, the permissions of the selected
822 tun(4) device must allow access to the user.
823 PermitUserEnvironment
825 Specifies whether ~/.ssh/environment and environment= options in
826 ~/.ssh/authorized_keys are processed by sshd(8). Valid options
827 are yes, no or a pattern-list specifying which environment vari‐
828 able names to accept (for example "LANG,LC_*"). The default is
829 no. Enabling environment processing may enable users to bypass
830 access restrictions in some configurations using mechanisms such
831 as LD_PRELOAD.
832 PermitUserRC
834 Specifies whether any ~/.ssh/rc file is executed. The default is
835 yes.
836 PidFile
838 Specifies the file that contains the process ID of the SSH dae‐
839 mon, or none to not write one. The default is
840 /var/run/gsisshd.pid.
841 Port Specifies the port number that sshd(8) listens on. The default
843 is 22. Multiple options of this type are permitted. See also
844 ListenAddress.
845 PrintLastLog
847 Specifies whether sshd(8) should print the date and time of the
848 last user login when a user logs in interactively. The default
849 is yes.
850 PrintMotd
852 Specifies whether sshd(8) should print /etc/motd when a user logs
853 in interactively. (On some systems it is also printed by the
854 shell, /etc/profile, or equivalent.) The default is yes.
855 PubkeyAcceptedKeyTypes
857 Specifies the key types that will be accepted for public key
858 authentication as a list of comma-separated patterns. Alter‐
859 nately if the specified value begins with a ‘+’ character, then
860 the specified key types will be appended to the default set
861 instead of replacing them. If the specified value begins with a
862 ‘-’ character, then the specified key types (including wildcards)
863 will be removed from the default set instead of replacing them.
864 The default is handled system-wide by crypto-policies(7). To see
865 the defaults and how to modifuy this default, see manual page
866 update-crypto-policies(8).
867 The list of available key types may also be obtained using "ssh
869 -Q key".
870 PubkeyAuthentication
872 Specifies whether public key authentication is allowed. The
873 default is yes.
874 RekeyLimit
876 Specifies the maximum amount of data that may be transmitted
877 before the session key is renegotiated, optionally followed a
878 maximum amount of time that may pass before the session key is
879 renegotiated. The first argument is specified in bytes and may
880 have a suffix of ‘K’, ‘M’, or ‘G’ to indicate Kilobytes,
881 Megabytes, or Gigabytes, respectively. The default is between
882 ‘1G’ and ‘4G’, depending on the cipher. The optional second
883 value is specified in seconds and may use any of the units docu‐
884 mented in the TIME FORMATS section. The default value for
885 RekeyLimit is default none, which means that rekeying is per‐
886 formed after the cipher's default amount of data has been sent or
887 received and no time based rekeying is done.
888 RevokedKeys
890 Specifies revoked public keys file, or none to not use one. Keys
891 listed in this file will be refused for public key authentica‐
892 tion. Note that if this file is not readable, then public key
893 authentication will be refused for all users. Keys may be speci‐
894 fied as a text file, listing one public key per line, or as an
895 OpenSSH Key Revocation List (KRL) as generated by ssh-keygen(1).
896 For more information on KRLs, see the KEY REVOCATION LISTS sec‐
897 tion in ssh-keygen(1).
898 RDomain
900 Specifies an explicit routing domain that is applied after
901 authentication has completed. The user session, as well and any
902 forwarded or listening IP sockets, will be bound to this
903 rdomain(4). If the routing domain is set to %D, then the domain
904 in which the incoming connection was received will be applied.
905 SetEnv Specifies one or more environment variables to set in child ses‐
907 sions started by sshd(8) as “NAME=VALUE”. The environment value
908 may be quoted (e.g. if it contains whitespace characters). Envi‐
909 ronment variables set by SetEnv override the default environment
910 and any variables specified by the user via AcceptEnv or
911 PermitUserEnvironment.
912 StreamLocalBindMask
914 Sets the octal file creation mode mask (umask) used when creating
915 a Unix-domain socket file for local or remote port forwarding.
916 This option is only used for port forwarding to a Unix-domain
917 socket file.
918 The default value is 0177, which creates a Unix-domain socket
920 file that is readable and writable only by the owner. Note that
921 not all operating systems honor the file mode on Unix-domain
922 socket files.
923 StreamLocalBindUnlink
925 Specifies whether to remove an existing Unix-domain socket file
926 for local or remote port forwarding before creating a new one.
927 If the socket file already exists and StreamLocalBindUnlink is
928 not enabled, sshd will be unable to forward the port to the Unix-
929 domain socket file. This option is only used for port forwarding
930 to a Unix-domain socket file.
931 The argument must be yes or no. The default is no.
933 StrictModes
935 Specifies whether sshd(8) should check file modes and ownership
936 of the user's files and home directory before accepting login.
937 This is normally desirable because novices sometimes accidentally
938 leave their directory or files world-writable. The default is
939 yes. Note that this does not apply to ChrootDirectory, whose
940 permissions and ownership are checked unconditionally.
941 Subsystem
943 Configures an external subsystem (e.g. file transfer daemon).
944 Arguments should be a subsystem name and a command (with optional
945 arguments) to execute upon subsystem request.
946 The command sftp-server implements the SFTP file transfer subsys‐
948 tem.
949 Alternately the name internal-sftp implements an in-process SFTP
951 server. This may simplify configurations using ChrootDirectory
952 to force a different filesystem root on clients.
953 By default no subsystems are defined.
955 SyslogFacility
957 Gives the facility code that is used when logging messages from
958 sshd(8). The possible values are: DAEMON, USER, AUTH, AUTHPRIV,
959 LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
960 The default is AUTH.
961 TCPKeepAlive
963 Specifies whether the system should send TCP keepalive messages
964 to the other side. If they are sent, death of the connection or
965 crash of one of the machines will be properly noticed. However,
966 this means that connections will die if the route is down tempo‐
967 rarily, and some people find it annoying. On the other hand, if
968 TCP keepalives are not sent, sessions may hang indefinitely on
969 the server, leaving "ghost" users and consuming server resources.
970 The default is yes (to send TCP keepalive messages), and the
972 server will notice if the network goes down or the client host
973 crashes. This avoids infinitely hanging sessions.
974 To disable TCP keepalive messages, the value should be set to no.
976 TrustedUserCAKeys
978 Specifies a file containing public keys of certificate authori‐
979 ties that are trusted to sign user certificates for authentica‐
980 tion, or none to not use one. Keys are listed one per line;
981 empty lines and comments starting with ‘#’ are allowed. If a
982 certificate is presented for authentication and has its signing
983 CA key listed in this file, then it may be used for authentica‐
984 tion for any user listed in the certificate's principals list.
985 Note that certificates that lack a list of principals will not be
986 permitted for authentication using TrustedUserCAKeys. For more
987 details on certificates, see the CERTIFICATES section in
988 ssh-keygen(1).
989 UseDNS Specifies whether sshd(8) should look up the remote host name,
991 and to check that the resolved host name for the remote IP
992 address maps back to the very same IP address.
993 If this option is set to no (the default) then only addresses and
995 not host names may be used in ~/.ssh/authorized_keys from and
996 sshd_config Match Host directives.
997 UsePAM Enables the Pluggable Authentication Module interface. If set to
999 yes this will enable PAM authentication using
1000 ChallengeResponseAuthentication and PasswordAuthentication in
1001 addition to PAM account and session module processing for all
1002 authentication types.
1003 Because PAM challenge-response authentication usually serves an
1005 equivalent role to password authentication, you should disable
1006 either PasswordAuthentication or ChallengeResponseAuthentication.
1007 If UsePAM is enabled, you will not be able to run sshd(8) as a
1009 non-root user. The default is no.
1010 PermitPAMUserChange
1012 If set to yes this will enable PAM authentication to change the
1013 name of the user being authenticated. The default is no.
1014 VersionAddendum
1016 Optionally specifies additional text to append to the SSH proto‐
1017 col banner sent by the server upon connection. The default is
1018 none.
1019 X11DisplayOffset
1021 Specifies the first display number available for sshd(8)'s X11
1022 forwarding. This prevents sshd from interfering with real X11
1023 servers. The default is 10.
1024 X11MaxDisplays
1026 Specifies the maximum number of displays available for sshd(8)'s
1027 X11 forwarding. This prevents sshd from exhausting local ports.
1028 The default is 1000.
1029 X11Forwarding
1031 Specifies whether X11 forwarding is permitted. The argument must
1032 be yes or no. The default is no.
1033 When X11 forwarding is enabled, there may be additional exposure
1035 to the server and to client displays if the sshd(8) proxy display
1036 is configured to listen on the wildcard address (see
1037 X11UseLocalhost), though this is not the default. Additionally,
1038 the authentication spoofing and authentication data verification
1039 and substitution occur on the client side. The security risk of
1040 using X11 forwarding is that the client's X11 display server may
1041 be exposed to attack when the SSH client requests forwarding (see
1042 the warnings for ForwardX11 in ssh_config(5)). A system adminis‐
1043 trator may have a stance in which they want to protect clients
1044 that may expose themselves to attack by unwittingly requesting
1045 X11 forwarding, which can warrant a no setting.
1046 Note that disabling X11 forwarding does not prevent users from
1048 forwarding X11 traffic, as users can always install their own
1049 forwarders.
1050 X11UseLocalhost
1052 Specifies whether sshd(8) should bind the X11 forwarding server
1053 to the loopback address or to the wildcard address. By default,
1054 sshd binds the forwarding server to the loopback address and sets
1055 the hostname part of the DISPLAY environment variable to
1056 localhost. This prevents remote hosts from connecting to the
1057 proxy display. However, some older X11 clients may not function
1058 with this configuration. X11UseLocalhost may be set to no to
1059 specify that the forwarding server should be bound to the wild‐
1060 card address. The argument must be yes or no. The default is
1061 yes.
1062 XAuthLocation
1064 Specifies the full pathname of the xauth(1) program, or none to
1065 not use one. The default is /usr/bin/xauth.
1066
1068 sshd(8) command-line arguments and configuration file options that spec‐
1069 ify time may be expressed using a sequence of the form: time[qualifier],
1070 where time is a positive integer value and qualifier is one of the fol‐
1071 lowing:
1072 ⟨none⟩ seconds
1074 s | S seconds
1075 m | M minutes
1076 h | H hours
1077 d | D days
1078 w | W weeks
1079 Each member of the sequence is added together to calculate the total time
1081 value.
1082 Time format examples:
1084 600 600 seconds (10 minutes)
1086 10m 10 minutes
1087 1h30m 1 hour 30 minutes (90 minutes)
1088
1090 Arguments to some keywords can make use of tokens, which are expanded at
1091 runtime:
1092 %% A literal ‘%’.
1094 %D The routing domain in which the incoming connection was
1095 received.
1096 %F The fingerprint of the CA key.
1097 %f The fingerprint of the key or certificate.
1098 %h The home directory of the user.
1099 %i The key ID in the certificate.
1100 %K The base64-encoded CA key.
1101 %k The base64-encoded key or certificate for authentication.
1102 %s The serial number of the certificate.
1103 %T The type of the CA key.
1104 %t The key or certificate type.
1105 %U The numeric user ID of the target user.
1106 %u The username.
1107 AuthorizedKeysCommand accepts the tokens %%, %f, %h, %k, %t, %U, and %u.
1109 AuthorizedKeysFile accepts the tokens %%, %h, %U, and %u.
1111 AuthorizedPrincipalsCommand accepts the tokens %%, %F, %f, %h, %i, %K,
1113 %k, %s, %T, %t, %U, and %u.
1114 AuthorizedPrincipalsFile accepts the tokens %%, %h, %U, and %u.
1116 ChrootDirectory accepts the tokens %%, %h, %U, and %u.
1118 RoutingDomain accepts the token %D.
1120
1122 /etc/gsissh/sshd_config
1123 Contains configuration data for sshd(8). This file should be
1124 writable by root only, but it is recommended (though not neces‐
1125 sary) that it be world-readable.
1126
1128 sftp-server(8), sshd(8)
1129
1131 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
1132 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
1133 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
1134 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
1135 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
1136 for privilege separation.
1137BSD June 21, 2019 BSD