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1SSHD_CONFIG(5) File Formats Manual SSHD_CONFIG(5)
2
3NAME
4 sshd_config M-bM-^@M-^S OpenSSH SSH daemon configuration file
5
6SYNOPSIS
7 /etc/ssh/sshd_config
8
9DESCRIPTION
10 sshd(8) reads configuration data from /etc/ssh/sshd_config (or the file
11 specified with -f on the command line). The file contains keyword-
12 argument pairs, one per line. Lines starting with M-bM-^@M-^X#M-bM-^@M-^Y and empty lines
13 are interpreted as comments. Arguments may optionally be enclosed in
14 double quotes (") in order to represent arguments containing spaces.
15
16 The possible keywords and their meanings are as follows (note that
17 keywords are case-insensitive and arguments are case-sensitive):
18
19 AcceptEnv
20 Specifies what environment variables sent by the client will be
21 copied into the session's environ(7). See SendEnv in
22 ssh_config(5) for how to configure the client. Note that
23 environment passing is only supported for protocol 2, and that
24 the TERM environment variable is always sent whenever the client
25 requests a pseudo-terminal as it is required by the protocol.
26 Variables are specified by name, which may contain the wildcard
27 characters M-bM-^@M-^X*M-bM-^@M-^Y and M-bM-^@M-^X?M-bM-^@M-^Y. Multiple environment variables may be
28 separated by whitespace or spread across multiple AcceptEnv
29 directives. Be warned that some environment variables could be
30 used to bypass restricted user environments. For this reason,
31 care should be taken in the use of this directive. The default
32 is not to accept any environment variables.
33
34 AddressFamily
35 Specifies which address family should be used by sshd(8). Valid
36 arguments are M-bM-^@M-^\anyM-bM-^@M-^], M-bM-^@M-^\inetM-bM-^@M-^] (use IPv4 only), or M-bM-^@M-^\inet6M-bM-^@M-^] (use IPv6
37 only). The default is M-bM-^@M-^\anyM-bM-^@M-^].
38
39 AllowAgentForwarding
40 Specifies whether ssh-agent(1) forwarding is permitted. The
41 default is M-bM-^@M-^\yesM-bM-^@M-^]. Note that disabling agent forwarding does not
42 improve security unless users are also denied shell access, as
43 they can always install their own forwarders.
44
45 AllowGroups
46 This keyword can be followed by a list of group name patterns,
47 separated by spaces. If specified, login is allowed only for
48 users whose primary group or supplementary group list matches one
49 of the patterns. Only group names are valid; a numerical group
50 ID is not recognized. By default, login is allowed for all
51 groups. The allow/deny directives are processed in the following
52 order: DenyUsers, AllowUsers, DenyGroups, and finally
53 AllowGroups.
54
55 See PATTERNS in ssh_config(5) for more information on patterns.
56
57 AllowTcpForwarding
58 Specifies whether TCP forwarding is permitted. The available
59 options are M-bM-^@M-^\yesM-bM-^@M-^] or M-bM-^@M-^\allM-bM-^@M-^] to allow TCP forwarding, M-bM-^@M-^\noM-bM-^@M-^] to
60 prevent all TCP forwarding, M-bM-^@M-^\localM-bM-^@M-^] to allow local (from the
61 perspective of ssh(1)) forwarding only or M-bM-^@M-^\remoteM-bM-^@M-^] to allow
62 remote forwarding only. The default is M-bM-^@M-^\yesM-bM-^@M-^]. Note that
63 disabling TCP forwarding does not improve security unless users
64 are also denied shell access, as they can always install their
65 own forwarders.
66
67 AllowStreamLocalForwarding
68 Specifies whether StreamLocal (Unix-domain socket) forwarding is
69 permitted. The available options are M-bM-^@M-^\yesM-bM-^@M-^] or M-bM-^@M-^\allM-bM-^@M-^] to allow
70 StreamLocal forwarding, M-bM-^@M-^\noM-bM-^@M-^] to prevent all StreamLocal
71 forwarding, M-bM-^@M-^\localM-bM-^@M-^] to allow local (from the perspective of
72 ssh(1)) forwarding only or M-bM-^@M-^\remoteM-bM-^@M-^] to allow remote forwarding
73 only. The default is M-bM-^@M-^\yesM-bM-^@M-^]. Note that disabling StreamLocal
74 forwarding does not improve security unless users are also denied
75 shell access, as they can always install their own forwarders.
76
77 AllowUsers
78 This keyword can be followed by a list of user name patterns,
79 separated by spaces. If specified, login is allowed only for
80 user names that match one of the patterns. Only user names are
81 valid; a numerical user ID is not recognized. By default, login
82 is allowed for all users. If the pattern takes the form
83 USER@HOST then USER and HOST are separately checked, restricting
84 logins to particular users from particular hosts. The allow/deny
85 directives are processed in the following order: DenyUsers,
86 AllowUsers, DenyGroups, and finally AllowGroups.
87
88 See PATTERNS in ssh_config(5) for more information on patterns.
89
90 AuthenticationMethods
91 Specifies the authentication methods that must be successfully
92 completed for a user to be granted access. This option must be
93 followed by one or more comma-separated lists of authentication
94 method names. Successful authentication requires completion of
95 every method in at least one of these lists.
96
97 For example, an argument of M-bM-^@M-^\publickey,password
98 publickey,keyboard-interactiveM-bM-^@M-^] would require the user to
99 complete public key authentication, followed by either password
100 or keyboard interactive authentication. Only methods that are
101 next in one or more lists are offered at each stage, so for this
102 example, it would not be possible to attempt password or
103 keyboard-interactive authentication before public key.
104
105 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 M-bM-^@M-^\bsdauthM-bM-^@M-^], M-bM-^@M-^\pamM-bM-^@M-^], or M-bM-^@M-^\skeyM-bM-^@M-^],
108 depending on the server configuration. For example,
109 M-bM-^@M-^\keyboard-interactive:bsdauthM-bM-^@M-^] would restrict keyboard
110 interactive authentication to the M-bM-^@M-^\bsdauthM-bM-^@M-^] device.
111
112 If the M-bM-^@M-^\publickeyM-bM-^@M-^] method is listed more than once, sshd(8)
113 verifies that keys that have been used successfully are not
114 reused for subsequent authentications. For example, an
115 AuthenticationMethods of M-bM-^@M-^\publickey,publickeyM-bM-^@M-^] will require
116 successful authentication using two different public keys.
117
118 This option is only available for SSH protocol 2 and will yield a
119 fatal error if enabled if protocol 1 is also enabled. Note that
120 each authentication method listed should also be explicitly
121 enabled in the configuration. The default is not to require
122 multiple authentication; successful completion of a single
123 authentication method is sufficient.
124
125 AuthorizedKeysCommand
126 Specifies a program to be used to look up the user's public keys.
127 The program must be owned by root, not writable by group or
128 others and specified by an absolute path.
129
130 Arguments to AuthorizedKeysCommand may be provided using the
131 following tokens, which will be expanded at runtime: %% is
132 replaced by a literal '%', %u is replaced by the username being
133 authenticated, %h is replaced by the home directory of the user
134 being authenticated, %t is replaced with the key type offered for
135 authentication, %f is replaced with the fingerprint of the key,
136 and %k is replaced with the key being offered for authentication.
137 If no arguments are specified then the username of the target
138 user will be supplied.
139
140 The program should produce on standard output zero or more lines
141 of authorized_keys output (see AUTHORIZED_KEYS in sshd(8)). If a
142 key supplied by AuthorizedKeysCommand does not successfully
143 authenticate and authorize the user then public key
144 authentication continues using the usual AuthorizedKeysFile
145 files. By default, no AuthorizedKeysCommand is run.
146
147 AuthorizedKeysCommandUser
148 Specifies the user under whose account the AuthorizedKeysCommand
149 is run. It is recommended to use a dedicated user that has no
150 other role on the host than running authorized keys commands. If
151 AuthorizedKeysCommand is specified but AuthorizedKeysCommandUser
152 is not, then sshd(8) will refuse to start.
153
154 AuthorizedKeysFile
155 Specifies the file that contains the public keys that can be used
156 for user authentication. The format is described in the
157 AUTHORIZED_KEYS FILE FORMAT section of sshd(8).
158 AuthorizedKeysFile may contain tokens of the form %T which are
159 substituted during connection setup. The following tokens are
160 defined: %% is replaced by a literal '%', %h is replaced by the
161 home directory of the user being authenticated, and %u is
162 replaced by the username of that user. After expansion,
163 AuthorizedKeysFile is taken to be an absolute path or one
164 relative to the user's home directory. Multiple files may be
165 listed, separated by whitespace. The default is
166 M-bM-^@M-^\.ssh/authorized_keys .ssh/authorized_keys2M-bM-^@M-^].
167
168 AuthorizedPrincipalsCommand
169 Specifies a program to be used to generate the list of allowed
170 certificate principals as per AuthorizedPrincipalsFile. The
171 program must be owned by root, not writable by group or others
172 and specified by an absolute path.
173
174 Arguments to AuthorizedPrincipalsCommand may be provided using
175 the following tokens, which will be expanded at runtime: %% is
176 replaced by a literal '%', %u is replaced by the username being
177 authenticated and %h is replaced by the home directory of the
178 user being authenticated.
179
180 The program should produce on standard output zero or more lines
181 of AuthorizedPrincipalsFile output. If either
182 AuthorizedPrincipalsCommand or AuthorizedPrincipalsFile is
183 specified, then certificates offered by the client for
184 authentication must contain a principal that is listed. By
185 default, no AuthorizedPrincipalsCommand is run.
186
187 AuthorizedPrincipalsCommandUser
188 Specifies the user under whose account the
189 AuthorizedPrincipalsCommand is run. It is recommended to use a
190 dedicated user that has no other role on the host than running
191 authorized principals commands. If AuthorizedPrincipalsCommand
192 is specified but AuthorizedPrincipalsCommandUser is not, then
193 sshd(8) will refuse to start.
194
195 AuthorizedPrincipalsFile
196 Specifies a file that lists principal names that are accepted for
197 certificate authentication. When using certificates signed by a
198 key listed in TrustedUserCAKeys, this file lists names, one of
199 which must appear in the certificate for it to be accepted for
200 authentication. Names are listed one per line preceded by key
201 options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)).
202 Empty lines and comments starting with M-bM-^@M-^X#M-bM-^@M-^Y are ignored.
203
204 AuthorizedPrincipalsFile may contain tokens of the form %T which
205 are substituted during connection setup. The following tokens
206 are defined: %% is replaced by a literal '%', %h is replaced by
207 the home directory of the user being authenticated, and %u is
208 replaced by the username of that user. After expansion,
209 AuthorizedPrincipalsFile is taken to be an absolute path or one
210 relative to the user's home directory.
211
212 The default is M-bM-^@M-^\noneM-bM-^@M-^], i.e. not to use a principals file M-bM-^@M-^S in
213 this case, the username of the user must appear in a
214 certificate's principals list for it to be accepted. Note that
215 AuthorizedPrincipalsFile is only used when authentication
216 proceeds using a CA listed in TrustedUserCAKeys and is not
217 consulted for certification authorities trusted via
218 ~/.ssh/authorized_keys, though the principals= key option offers
219 a similar facility (see sshd(8) for details).
220
221 Banner The contents of the specified file are sent to the remote user
222 before authentication is allowed. If the argument is M-bM-^@M-^\noneM-bM-^@M-^] then
223 no banner is displayed. This option is only available for
224 protocol version 2. By default, no banner is displayed.
225
226 ChallengeResponseAuthentication
227 Specifies whether challenge-response authentication is allowed
228 (e.g. via PAM or through authentication styles supported in
229 login.conf(5)) The default is M-bM-^@M-^\yesM-bM-^@M-^].
230
231 ChrootDirectory
232 Specifies the pathname of a directory to chroot(2) to after
233 authentication. At session startup sshd(8) checks that all
234 components of the pathname are root-owned directories which are
235 not writable by any other user or group. After the chroot,
236 sshd(8) changes the working directory to the user's home
237 directory.
238
239 The pathname may contain the following tokens that are expanded
240 at runtime once the connecting user has been authenticated: %% is
241 replaced by a literal '%', %h is replaced by the home directory
242 of the user being authenticated, and %u is replaced by the
243 username of that user.
244
245 The ChrootDirectory must contain the necessary files and
246 directories to support the user's session. For an interactive
247 session this requires at least a shell, typically sh(1), and
248 basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4),
249 stderr(4), and tty(4) devices. For file transfer sessions using
250 M-bM-^@M-^\sftpM-bM-^@M-^], no additional configuration of the environment is
251 necessary if the in-process sftp server is used, though sessions
252 which use logging may require /dev/log inside the chroot
253 directory on some operating systems (see sftp-server(8) for
254 details).
255
256 For safety, it is very important that the directory hierarchy be
257 prevented from modification by other processes on the system
258 (especially those outside the jail). Misconfiguration can lead
259 to unsafe environments which sshd(8) cannot detect.
260
261 The default is not to chroot(2).
262
263 Ciphers
264 Specifies the ciphers allowed for protocol version 2. Multiple
265 ciphers must be comma-separated. If the specified value begins
266 with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the specified ciphers will be appended
267 to the default set instead of replacing them.
268
269 The supported ciphers are:
270
271 3des-cbc
272 aes128-cbc
273 aes192-cbc
274 aes256-cbc
275 aes128-ctr
276 aes192-ctr
277 aes256-ctr
278 aes128-gcm@openssh.com
279 aes256-gcm@openssh.com
280 arcfour
281 arcfour128
282 arcfour256
283 blowfish-cbc
284 cast128-cbc
285 chacha20-poly1305@openssh.com
286
287 The default is:
288
289 aes128-ctr,aes192-ctr,aes256-ctr,
290 aes128-gcm@openssh.com,aes256-gcm@openssh.com,
291 chacha20-poly1305@openssh.com
292
293 The list of available ciphers may also be obtained using the -Q
294 option of ssh(1) with an argument of M-bM-^@M-^\cipherM-bM-^@M-^].
295
296 ClientAliveCountMax
297 Sets the number of client alive messages (see below) which may be
298 sent without sshd(8) receiving any messages back from the client.
299 If this threshold is reached while client alive messages are
300 being sent, sshd will disconnect the client, terminating the
301 session. It is important to note that the use of client alive
302 messages is very different from TCPKeepAlive (below). The client
303 alive messages are sent through the encrypted channel and
304 therefore will not be spoofable. The TCP keepalive option
305 enabled by TCPKeepAlive is spoofable. The client alive mechanism
306 is valuable when the client or server depend on knowing when a
307 connection has become inactive.
308
309 The default value is 3. If ClientAliveInterval (see below) is
310 set to 15, and ClientAliveCountMax is left at the default,
311 unresponsive SSH clients will be disconnected after approximately
312 45 seconds. This option applies to protocol version 2 only.
313
314 ClientAliveInterval
315 Sets a timeout interval in seconds after which if no data has
316 been received from the client, sshd(8) will send a message
317 through the encrypted channel to request a response from the
318 client. The default is 0, indicating that these messages will
319 not be sent to the client. This option applies to protocol
320 version 2 only.
321
322 Compression
323 Specifies whether compression is allowed, or delayed until the
324 user has authenticated successfully. The argument must be M-bM-^@M-^\yesM-bM-^@M-^],
325 M-bM-^@M-^\delayedM-bM-^@M-^], or M-bM-^@M-^\noM-bM-^@M-^]. The default is M-bM-^@M-^\delayedM-bM-^@M-^].
326
327 DenyGroups
328 This keyword can be followed by a list of group name patterns,
329 separated by spaces. Login is disallowed for users whose primary
330 group or supplementary group list matches one of the patterns.
331 Only group names are valid; a numerical group ID is not
332 recognized. By default, login is allowed for all groups. The
333 allow/deny directives are processed in the following order:
334 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
335
336 See PATTERNS in ssh_config(5) for more information on patterns.
337
338 DenyUsers
339 This keyword can be followed by a list of user name patterns,
340 separated by spaces. Login is disallowed for user names that
341 match one of the patterns. Only user names are valid; a
342 numerical user ID is not recognized. By default, login is
343 allowed for all users. If the pattern takes the form USER@HOST
344 then USER and HOST are separately checked, restricting logins to
345 particular users from particular hosts. The allow/deny
346 directives are processed in the following order: DenyUsers,
347 AllowUsers, DenyGroups, and finally AllowGroups.
348
349 See PATTERNS in ssh_config(5) for more information on patterns.
350
351 FingerprintHash
352 Specifies the hash algorithm used when logging key fingerprints.
353 Valid options are: M-bM-^@M-^\md5M-bM-^@M-^] and M-bM-^@M-^\sha256M-bM-^@M-^]. The default is M-bM-^@M-^\sha256M-bM-^@M-^].
354
355 ForceCommand
356 Forces the execution of the command specified by ForceCommand,
357 ignoring any command supplied by the client and ~/.ssh/rc if
358 present. The command is invoked by using the user's login shell
359 with the -c option. This applies to shell, command, or subsystem
360 execution. It is most useful inside a Match block. The command
361 originally supplied by the client is available in the
362 SSH_ORIGINAL_COMMAND environment variable. Specifying a command
363 of M-bM-^@M-^\internal-sftpM-bM-^@M-^] will force the use of an in-process sftp
364 server that requires no support files when used with
365 ChrootDirectory.
366
367 GatewayPorts
368 Specifies whether remote hosts are allowed to connect to ports
369 forwarded for the client. By default, sshd(8) binds remote port
370 forwardings to the loopback address. This prevents other remote
371 hosts from connecting to forwarded ports. GatewayPorts can be
372 used to specify that sshd should allow remote port forwardings to
373 bind to non-loopback addresses, thus allowing other hosts to
374 connect. The argument may be M-bM-^@M-^\noM-bM-^@M-^] to force remote port
375 forwardings to be available to the local host only, M-bM-^@M-^\yesM-bM-^@M-^] to
376 force remote port forwardings to bind to the wildcard address, or
377 M-bM-^@M-^\clientspecifiedM-bM-^@M-^] to allow the client to select the address to
378 which the forwarding is bound. The default is M-bM-^@M-^\noM-bM-^@M-^].
379
380 GSSAPIAuthentication
381 Specifies whether user authentication based on GSSAPI is allowed.
382 The default is M-bM-^@M-^\noM-bM-^@M-^]. Note that this option applies to protocol
383 version 2 only.
384
385 GSSAPICleanupCredentials
386 Specifies whether to automatically destroy the user's credentials
387 cache on logout. The default is M-bM-^@M-^\yesM-bM-^@M-^]. Note that this option
388 applies to protocol version 2 only.
389
390 GSSAPIStrictAcceptorCheck
391 Determines whether to be strict about the identity of the GSSAPI
392 acceptor a client authenticates against. If set to M-bM-^@M-^\yesM-bM-^@M-^] then
393 the client must authenticate against the host service on the
394 current hostname. If set to M-bM-^@M-^\noM-bM-^@M-^] then the client may
395 authenticate against any service key stored in the machine's
396 default store. This facility is provided to assist with
397 operation on multi homed machines. The default is M-bM-^@M-^\yesM-bM-^@M-^].
398
399 HostbasedAcceptedKeyTypes
400 Specifies the key types that will be accepted for hostbased
401 authentication as a comma-separated pattern list. Alternately if
402 the specified value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the
403 specified key types will be appended to the default set instead
404 of replacing them. The default for this option is:
405
406 ecdsa-sha2-nistp256-cert-v01@openssh.com,
407 ecdsa-sha2-nistp384-cert-v01@openssh.com,
408 ecdsa-sha2-nistp521-cert-v01@openssh.com,
409 ssh-ed25519-cert-v01@openssh.com,
410 ssh-rsa-cert-v01@openssh.com,
411 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
412 ssh-ed25519,ssh-rsa
413
414 The -Q option of ssh(1) may be used to list supported key types.
415
416 HostbasedAuthentication
417 Specifies whether rhosts or /etc/hosts.equiv authentication
418 together with successful public key client host authentication is
419 allowed (host-based authentication). This option is similar to
420 RhostsRSAAuthentication and applies to protocol version 2 only.
421 The default is M-bM-^@M-^\noM-bM-^@M-^].
422
423 HostbasedUsesNameFromPacketOnly
424 Specifies whether or not the server will attempt to perform a
425 reverse name lookup when matching the name in the ~/.shosts,
426 ~/.rhosts, and /etc/hosts.equiv files during
427 HostbasedAuthentication. A setting of M-bM-^@M-^\yesM-bM-^@M-^] means that sshd(8)
428 uses the name supplied by the client rather than attempting to
429 resolve the name from the TCP connection itself. The default is
430 M-bM-^@M-^\noM-bM-^@M-^].
431
432 HostCertificate
433 Specifies a file containing a public host certificate. The
434 certificate's public key must match a private host key already
435 specified by HostKey. The default behaviour of sshd(8) is not to
436 load any certificates.
437
438 HostKey
439 Specifies a file containing a private host key used by SSH. The
440 default is /etc/ssh/ssh_host_key for protocol version 1, and
441 /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key,
442 /etc/ssh/ssh_host_ed25519_key and /etc/ssh/ssh_host_rsa_key for
443 protocol version 2.
444
445 Note that sshd(8) will refuse to use a file if it is group/world-
446 accessible and that the HostKeyAlgorithms option restricts which
447 of the keys are actually used by sshd(8).
448
449 It is possible to have multiple host key files. M-bM-^@M-^\rsa1M-bM-^@M-^] keys are
450 used for version 1 and M-bM-^@M-^\dsaM-bM-^@M-^], M-bM-^@M-^\ecdsaM-bM-^@M-^], M-bM-^@M-^\ed25519M-bM-^@M-^] or M-bM-^@M-^\rsaM-bM-^@M-^] are
451 used for version 2 of the SSH protocol. It is also possible to
452 specify public host key files instead. In this case operations
453 on the private key will be delegated to an ssh-agent(1).
454
455 HostKeyAgent
456 Identifies the UNIX-domain socket used to communicate with an
457 agent that has access to the private host keys. If
458 M-bM-^@M-^\SSH_AUTH_SOCKM-bM-^@M-^] is specified, the location of the socket will be
459 read from the SSH_AUTH_SOCK environment variable.
460
461 HostKeyAlgorithms
462 Specifies the protocol version 2 host key algorithms that the
463 server offers. The default for this option is:
464
465 ecdsa-sha2-nistp256-cert-v01@openssh.com,
466 ecdsa-sha2-nistp384-cert-v01@openssh.com,
467 ecdsa-sha2-nistp521-cert-v01@openssh.com,
468 ssh-ed25519-cert-v01@openssh.com,
469 ssh-rsa-cert-v01@openssh.com,
470 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
471 ssh-ed25519,ssh-rsa
472
473 The list of available key types may also be obtained using the -Q
474 option of ssh(1) with an argument of M-bM-^@M-^\keyM-bM-^@M-^].
475
476 IgnoreRhosts
477 Specifies that .rhosts and .shosts files will not be used in
478 RhostsRSAAuthentication or HostbasedAuthentication.
479
480 /etc/hosts.equiv and /etc/shosts.equiv are still used. The
481 default is M-bM-^@M-^\yesM-bM-^@M-^].
482
483 IgnoreUserKnownHosts
484 Specifies whether sshd(8) should ignore the user's
485 ~/.ssh/known_hosts during RhostsRSAAuthentication or
486 HostbasedAuthentication. The default is M-bM-^@M-^\noM-bM-^@M-^].
487
488 IPQoS Specifies the IPv4 type-of-service or DSCP class for the
489 connection. Accepted values are M-bM-^@M-^\af11M-bM-^@M-^], M-bM-^@M-^\af12M-bM-^@M-^], M-bM-^@M-^\af13M-bM-^@M-^], M-bM-^@M-^\af21M-bM-^@M-^],
490 M-bM-^@M-^\af22M-bM-^@M-^], M-bM-^@M-^\af23M-bM-^@M-^], M-bM-^@M-^\af31M-bM-^@M-^], M-bM-^@M-^\af32M-bM-^@M-^], M-bM-^@M-^\af33M-bM-^@M-^], M-bM-^@M-^\af41M-bM-^@M-^], M-bM-^@M-^\af42M-bM-^@M-^], M-bM-^@M-^\af43M-bM-^@M-^],
491 M-bM-^@M-^\cs0M-bM-^@M-^], M-bM-^@M-^\cs1M-bM-^@M-^], M-bM-^@M-^\cs2M-bM-^@M-^], M-bM-^@M-^\cs3M-bM-^@M-^], M-bM-^@M-^\cs4M-bM-^@M-^], M-bM-^@M-^\cs5M-bM-^@M-^], M-bM-^@M-^\cs6M-bM-^@M-^], M-bM-^@M-^\cs7M-bM-^@M-^], M-bM-^@M-^\efM-bM-^@M-^],
492 M-bM-^@M-^\lowdelayM-bM-^@M-^], M-bM-^@M-^\throughputM-bM-^@M-^], M-bM-^@M-^\reliabilityM-bM-^@M-^], or a numeric value.
493 This option may take one or two arguments, separated by
494 whitespace. If one argument is specified, it is used as the
495 packet class unconditionally. If two values are specified, the
496 first is automatically selected for interactive sessions and the
497 second for non-interactive sessions. The default is M-bM-^@M-^\lowdelayM-bM-^@M-^]
498 for interactive sessions and M-bM-^@M-^\throughputM-bM-^@M-^] for non-interactive
499 sessions.
500
501 KbdInteractiveAuthentication
502 Specifies whether to allow keyboard-interactive authentication.
503 The argument to this keyword must be M-bM-^@M-^\yesM-bM-^@M-^] or M-bM-^@M-^\noM-bM-^@M-^]. The default
504 is to use whatever value ChallengeResponseAuthentication is set
505 to (by default M-bM-^@M-^\yesM-bM-^@M-^]).
506
507 KerberosAuthentication
508 Specifies whether the password provided by the user for
509 PasswordAuthentication will be validated through the Kerberos
510 KDC. To use this option, the server needs a Kerberos servtab
511 which allows the verification of the KDC's identity. The default
512 is M-bM-^@M-^\noM-bM-^@M-^].
513
514 KerberosGetAFSToken
515 If AFS is active and the user has a Kerberos 5 TGT, attempt to
516 acquire an AFS token before accessing the user's home directory.
517 The default is M-bM-^@M-^\noM-bM-^@M-^].
518
519 KerberosOrLocalPasswd
520 If password authentication through Kerberos fails then the
521 password will be validated via any additional local mechanism
522 such as /etc/passwd. The default is M-bM-^@M-^\yesM-bM-^@M-^].
523
524 KerberosTicketCleanup
525 Specifies whether to automatically destroy the user's ticket
526 cache file on logout. The default is M-bM-^@M-^\yesM-bM-^@M-^].
527
528 KexAlgorithms
529 Specifies the available KEX (Key Exchange) algorithms. Multiple
530 algorithms must be comma-separated. Alternately if the specified
531 value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the specified methods
532 will be appended to the default set instead of replacing them.
533 The supported algorithms are:
534
535 curve25519-sha256@libssh.org
536 diffie-hellman-group1-sha1
537 diffie-hellman-group14-sha1
538 diffie-hellman-group-exchange-sha1
539 diffie-hellman-group-exchange-sha256
540 ecdh-sha2-nistp256
541 ecdh-sha2-nistp384
542 ecdh-sha2-nistp521
543
544 The default is:
545
546 curve25519-sha256@libssh.org,
547 ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,
548 diffie-hellman-group-exchange-sha256,
549 diffie-hellman-group14-sha1
550
551 The list of available key exchange algorithms may also be
552 obtained using the -Q option of ssh(1) with an argument of M-bM-^@M-^\kexM-bM-^@M-^].
553
554 KeyRegenerationInterval
555 In protocol version 1, the ephemeral server key is automatically
556 regenerated after this many seconds (if it has been used). The
557 purpose of regeneration is to prevent decrypting captured
558 sessions by later breaking into the machine and stealing the
559 keys. The key is never stored anywhere. If the value is 0, the
560 key is never regenerated. The default is 3600 (seconds).
561
562 ListenAddress
563 Specifies the local addresses sshd(8) should listen on. The
564 following forms may be used:
565
566 ListenAddress host|IPv4_addr|IPv6_addr
567 ListenAddress host|IPv4_addr:port
568 ListenAddress [host|IPv6_addr]:port
569
570 If port is not specified, sshd will listen on the address and all
571 Port options specified. The default is to listen on all local
572 addresses. Multiple ListenAddress options are permitted.
573
574 LoginGraceTime
575 The server disconnects after this time if the user has not
576 successfully logged in. If the value is 0, there is no time
577 limit. The default is 120 seconds.
578
579 LogLevel
580 Gives the verbosity level that is used when logging messages from
581 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO,
582 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
583 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
584 higher levels of debugging output. Logging with a DEBUG level
585 violates the privacy of users and is not recommended.
586
587 MACs Specifies the available MAC (message authentication code)
588 algorithms. The MAC algorithm is used in protocol version 2 for
589 data integrity protection. Multiple algorithms must be comma-
590 separated. If the specified value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character,
591 then the specified algorithms will be appended to the default set
592 instead of replacing them.
593
594 The algorithms that contain M-bM-^@M-^\-etmM-bM-^@M-^] calculate the MAC after
595 encryption (encrypt-then-mac). These are considered safer and
596 their use recommended. The supported MACs are:
597
598 hmac-md5
599 hmac-md5-96
600 hmac-ripemd160
601 hmac-sha1
602 hmac-sha1-96
603 hmac-sha2-256
604 hmac-sha2-512
605 umac-64@openssh.com
606 umac-128@openssh.com
607 hmac-md5-etm@openssh.com
608 hmac-md5-96-etm@openssh.com
609 hmac-ripemd160-etm@openssh.com
610 hmac-sha1-etm@openssh.com
611 hmac-sha1-96-etm@openssh.com
612 hmac-sha2-256-etm@openssh.com
613 hmac-sha2-512-etm@openssh.com
614 umac-64-etm@openssh.com
615 umac-128-etm@openssh.com
616
617 The default is:
618
619 umac-64-etm@openssh.com,umac-128-etm@openssh.com,
620 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,
621 umac-64@openssh.com,umac-128@openssh.com,
622 hmac-sha2-256,hmac-sha2-512
623
624 The list of available MAC algorithms may also be obtained using
625 the -Q option of ssh(1) with an argument of M-bM-^@M-^\macM-bM-^@M-^].
626
627 Match Introduces a conditional block. If all of the criteria on the
628 Match line are satisfied, the keywords on the following lines
629 override those set in the global section of the config file,
630 until either another Match line or the end of the file. If a
631 keyword appears in multiple Match blocks that are satisfied, only
632 the first instance of the keyword is applied.
633
634 The arguments to Match are one or more criteria-pattern pairs or
635 the single token All which matches all criteria. The available
636 criteria are User, Group, Host, LocalAddress, LocalPort, and
637 Address. The match patterns may consist of single entries or
638 comma-separated lists and may use the wildcard and negation
639 operators described in the PATTERNS section of ssh_config(5).
640
641 The patterns in an Address criteria may additionally contain
642 addresses to match in CIDR address/masklen format, e.g.
643 M-bM-^@M-^\192.0.2.0/24M-bM-^@M-^] or M-bM-^@M-^\3ffe:ffff::/32M-bM-^@M-^]. Note that the mask length
644 provided must be consistent with the address - it is an error to
645 specify a mask length that is too long for the address or one
646 with bits set in this host portion of the address. For example,
647 M-bM-^@M-^\192.0.2.0/33M-bM-^@M-^] and M-bM-^@M-^\192.0.2.0/8M-bM-^@M-^] respectively.
648
649 Only a subset of keywords may be used on the lines following a
650 Match keyword. Available keywords are AcceptEnv,
651 AllowAgentForwarding, AllowGroups, AllowStreamLocalForwarding,
652 AllowTcpForwarding, AllowUsers, AuthenticationMethods,
653 AuthorizedKeysCommand, AuthorizedKeysCommandUser,
654 AuthorizedKeysFile, AuthorizedPrincipalsFile, Banner,
655 ChrootDirectory, DenyGroups, DenyUsers, ForceCommand,
656 GatewayPorts, GSSAPIAuthentication, HostbasedAcceptedKeyTypes,
657 HostbasedAuthentication, HostbasedUsesNameFromPacketOnly, IPQoS,
658 KbdInteractiveAuthentication, KerberosAuthentication,
659 MaxAuthTries, MaxSessions, PasswordAuthentication,
660 PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTTY,
661 PermitTunnel, PermitUserRC, PubkeyAcceptedKeyTypes,
662 PubkeyAuthentication, RekeyLimit, RevokedKeys,
663 RhostsRSAAuthentication, RSAAuthentication, StreamLocalBindMask,
664 StreamLocalBindUnlink, TrustedUserCAKeys, X11DisplayOffset,
665 X11Forwarding and X11UseLocalHost.
666
667 MaxAuthTries
668 Specifies the maximum number of authentication attempts permitted
669 per connection. Once the number of failures reaches half this
670 value, additional failures are logged. The default is 6.
671
672 MaxSessions
673 Specifies the maximum number of open sessions permitted per
674 network connection. The default is 10.
675
676 MaxStartups
677 Specifies the maximum number of concurrent unauthenticated
678 connections to the SSH daemon. Additional connections will be
679 dropped until authentication succeeds or the LoginGraceTime
680 expires for a connection. The default is 10:30:100.
681
682 Alternatively, random early drop can be enabled by specifying the
683 three colon separated values M-bM-^@M-^\start:rate:fullM-bM-^@M-^] (e.g. "10:30:60").
684 sshd(8) will refuse connection attempts with a probability of
685 M-bM-^@M-^\rate/100M-bM-^@M-^] (30%) if there are currently M-bM-^@M-^\startM-bM-^@M-^] (10)
686 unauthenticated connections. The probability increases linearly
687 and all connection attempts are refused if the number of
688 unauthenticated connections reaches M-bM-^@M-^\fullM-bM-^@M-^] (60).
689
690 PasswordAuthentication
691 Specifies whether password authentication is allowed. The
692 default is M-bM-^@M-^\yesM-bM-^@M-^].
693
694 PermitEmptyPasswords
695 When password authentication is allowed, it specifies whether the
696 server allows login to accounts with empty password strings. The
697 default is M-bM-^@M-^\noM-bM-^@M-^].
698
699 PermitOpen
700 Specifies the destinations to which TCP port forwarding is
701 permitted. The forwarding specification must be one of the
702 following forms:
703
704 PermitOpen host:port
705 PermitOpen IPv4_addr:port
706 PermitOpen [IPv6_addr]:port
707
708 Multiple forwards may be specified by separating them with
709 whitespace. An argument of M-bM-^@M-^\anyM-bM-^@M-^] can be used to remove all
710 restrictions and permit any forwarding requests. An argument of
711 M-bM-^@M-^\noneM-bM-^@M-^] can be used to prohibit all forwarding requests. By
712 default all port forwarding requests are permitted.
713
714 PermitRootLogin
715 Specifies whether root can log in using ssh(1). The argument
716 must be M-bM-^@M-^\yesM-bM-^@M-^], M-bM-^@M-^\prohibit-passwordM-bM-^@M-^], M-bM-^@M-^\without-passwordM-bM-^@M-^],
717 M-bM-^@M-^\forced-commands-onlyM-bM-^@M-^], or M-bM-^@M-^\noM-bM-^@M-^]. The default is
718 M-bM-^@M-^\prohibit-passwordM-bM-^@M-^].
719
720 If this option is set to M-bM-^@M-^\prohibit-passwordM-bM-^@M-^] or
721 M-bM-^@M-^\without-passwordM-bM-^@M-^], password and keyboard-interactive
722 authentication are disabled for root.
723
724 If this option is set to M-bM-^@M-^\forced-commands-onlyM-bM-^@M-^], root login with
725 public key authentication will be allowed, but only if the
726 command option has been specified (which may be useful for taking
727 remote backups even if root login is normally not allowed). All
728 other authentication methods are disabled for root.
729
730 If this option is set to M-bM-^@M-^\noM-bM-^@M-^], root is not allowed to log in.
731
732 PermitTunnel
733 Specifies whether tun(4) device forwarding is allowed. The
734 argument must be M-bM-^@M-^\yesM-bM-^@M-^], M-bM-^@M-^\point-to-pointM-bM-^@M-^] (layer 3), M-bM-^@M-^\ethernetM-bM-^@M-^]
735 (layer 2), or M-bM-^@M-^\noM-bM-^@M-^]. Specifying M-bM-^@M-^\yesM-bM-^@M-^] permits both
736 M-bM-^@M-^\point-to-pointM-bM-^@M-^] and M-bM-^@M-^\ethernetM-bM-^@M-^]. The default is M-bM-^@M-^\noM-bM-^@M-^].
737
738 Independent of this setting, the permissions of the selected
739 tun(4) device must allow access to the user.
740
741 PermitTTY
742 Specifies whether pty(4) allocation is permitted. The default is
743 M-bM-^@M-^\yesM-bM-^@M-^].
744
745 PermitUserEnvironment
746 Specifies whether ~/.ssh/environment and environment= options in
747 ~/.ssh/authorized_keys are processed by sshd(8). The default is
748 M-bM-^@M-^\noM-bM-^@M-^]. Enabling environment processing may enable users to bypass
749 access restrictions in some configurations using mechanisms such
750 as LD_PRELOAD.
751
752 PermitUserRC
753 Specifies whether any ~/.ssh/rc file is executed. The default is
754 M-bM-^@M-^\yesM-bM-^@M-^].
755
756 PidFile
757 Specifies the file that contains the process ID of the SSH
758 daemon, or M-bM-^@M-^\noneM-bM-^@M-^] to not write one. The default is
759 /var/run/sshd.pid.
760
761 Port Specifies the port number that sshd(8) listens on. The default
762 is 22. Multiple options of this type are permitted. See also
763 ListenAddress.
764
765 PrintLastLog
766 Specifies whether sshd(8) should print the date and time of the
767 last user login when a user logs in interactively. The default
768 is M-bM-^@M-^\yesM-bM-^@M-^].
769
770 PrintMotd
771 Specifies whether sshd(8) should print /etc/motd when a user logs
772 in interactively. (On some systems it is also printed by the
773 shell, /etc/profile, or equivalent.) The default is M-bM-^@M-^\yesM-bM-^@M-^].
774
775 Protocol
776 Specifies the protocol versions sshd(8) supports. The possible
777 values are M-bM-^@M-^X1M-bM-^@M-^Y and M-bM-^@M-^X2M-bM-^@M-^Y. Multiple versions must be comma-
778 separated. The default is M-bM-^@M-^X2M-bM-^@M-^Y. Note that the order of the
779 protocol list does not indicate preference, because the client
780 selects among multiple protocol versions offered by the server.
781 Specifying M-bM-^@M-^\2,1M-bM-^@M-^] is identical to M-bM-^@M-^\1,2M-bM-^@M-^].
782
783 PubkeyAcceptedKeyTypes
784 Specifies the key types that will be accepted for public key
785 authentication as a comma-separated pattern list. Alternately if
786 the specified value begins with a M-bM-^@M-^X+M-bM-^@M-^Y character, then the
787 specified key types will be appended to the default set instead
788 of replacing them. The default for this option is:
789
790 ecdsa-sha2-nistp256-cert-v01@openssh.com,
791 ecdsa-sha2-nistp384-cert-v01@openssh.com,
792 ecdsa-sha2-nistp521-cert-v01@openssh.com,
793 ssh-ed25519-cert-v01@openssh.com,
794 ssh-rsa-cert-v01@openssh.com,
795 ecdsa-sha2-nistp256,ecdsa-sha2-nistp384,ecdsa-sha2-nistp521,
796 ssh-ed25519,ssh-rsa
797
798 The -Q option of ssh(1) may be used to list supported key types.
799
800 PubkeyAuthentication
801 Specifies whether public key authentication is allowed. The
802 default is M-bM-^@M-^\yesM-bM-^@M-^]. Note that this option applies to protocol
803 version 2 only.
804
805 RekeyLimit
806 Specifies the maximum amount of data that may be transmitted
807 before the session key is renegotiated, optionally followed a
808 maximum amount of time that may pass before the session key is
809 renegotiated. The first argument is specified in bytes and may
810 have a suffix of M-bM-^@M-^XKM-bM-^@M-^Y, M-bM-^@M-^XMM-bM-^@M-^Y, or M-bM-^@M-^XGM-bM-^@M-^Y to indicate Kilobytes,
811 Megabytes, or Gigabytes, respectively. The default is between
812 M-bM-^@M-^X1GM-bM-^@M-^Y and M-bM-^@M-^X4GM-bM-^@M-^Y, depending on the cipher. The optional second
813 value is specified in seconds and may use any of the units
814 documented in the TIME FORMATS section. The default value for
815 RekeyLimit is M-bM-^@M-^\default noneM-bM-^@M-^], which means that rekeying is
816 performed after the cipher's default amount of data has been sent
817 or received and no time based rekeying is done. This option
818 applies to protocol version 2 only.
819
820 RevokedKeys
821 Specifies revoked public keys file, or M-bM-^@M-^\noneM-bM-^@M-^] to not use one.
822 Keys listed in this file will be refused for public key
823 authentication. Note that if this file is not readable, then
824 public key authentication will be refused for all users. Keys
825 may be specified as a text file, listing one public key per line,
826 or as an OpenSSH Key Revocation List (KRL) as generated by
827 ssh-keygen(1). For more information on KRLs, see the KEY
828 REVOCATION LISTS section in ssh-keygen(1).
829
830 RhostsRSAAuthentication
831 Specifies whether rhosts or /etc/hosts.equiv authentication
832 together with successful RSA host authentication is allowed. The
833 default is M-bM-^@M-^\noM-bM-^@M-^]. This option applies to protocol version 1 only.
834
835 RSAAuthentication
836 Specifies whether pure RSA authentication is allowed. The
837 default is M-bM-^@M-^\yesM-bM-^@M-^]. This option applies to protocol version 1
838 only.
839
840 ServerKeyBits
841 Defines the number of bits in the ephemeral protocol version 1
842 server key. The default and minimum value is 1024.
843
844 StreamLocalBindMask
845 Sets the octal file creation mode mask (umask) used when creating
846 a Unix-domain socket file for local or remote port forwarding.
847 This option is only used for port forwarding to a Unix-domain
848 socket file.
849
850 The default value is 0177, which creates a Unix-domain socket
851 file that is readable and writable only by the owner. Note that
852 not all operating systems honor the file mode on Unix-domain
853 socket files.
854
855 StreamLocalBindUnlink
856 Specifies whether to remove an existing Unix-domain socket file
857 for local or remote port forwarding before creating a new one.
858 If the socket file already exists and StreamLocalBindUnlink is
859 not enabled, sshd will be unable to forward the port to the Unix-
860 domain socket file. This option is only used for port forwarding
861 to a Unix-domain socket file.
862
863 The argument must be M-bM-^@M-^\yesM-bM-^@M-^] or M-bM-^@M-^\noM-bM-^@M-^]. The default is M-bM-^@M-^\noM-bM-^@M-^].
864
865 StrictModes
866 Specifies whether sshd(8) should check file modes and ownership
867 of the user's files and home directory before accepting login.
868 This is normally desirable because novices sometimes accidentally
869 leave their directory or files world-writable. The default is
870 M-bM-^@M-^\yesM-bM-^@M-^]. Note that this does not apply to ChrootDirectory, whose
871 permissions and ownership are checked unconditionally.
872
873 Subsystem
874 Configures an external subsystem (e.g. file transfer daemon).
875 Arguments should be a subsystem name and a command (with optional
876 arguments) to execute upon subsystem request.
877
878 The command sftp-server(8) implements the M-bM-^@M-^\sftpM-bM-^@M-^] file transfer
879 subsystem.
880
881 Alternately the name M-bM-^@M-^\internal-sftpM-bM-^@M-^] implements an in-process
882 M-bM-^@M-^\sftpM-bM-^@M-^] server. This may simplify configurations using
883 ChrootDirectory to force a different filesystem root on clients.
884
885 By default no subsystems are defined. Note that this option
886 applies to protocol version 2 only.
887
888 SyslogFacility
889 Gives the facility code that is used when logging messages from
890 sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0,
891 LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The
892 default is AUTH.
893
894 TCPKeepAlive
895 Specifies whether the system should send TCP keepalive messages
896 to the other side. If they are sent, death of the connection or
897 crash of one of the machines will be properly noticed. However,
898 this means that connections will die if the route is down
899 temporarily, and some people find it annoying. On the other
900 hand, if TCP keepalives are not sent, sessions may hang
901 indefinitely on the server, leaving M-bM-^@M-^\ghostM-bM-^@M-^] users and consuming
902 server resources.
903
904 The default is M-bM-^@M-^\yesM-bM-^@M-^] (to send TCP keepalive messages), and the
905 server will notice if the network goes down or the client host
906 crashes. This avoids infinitely hanging sessions.
907
908 To disable TCP keepalive messages, the value should be set to
909 M-bM-^@M-^\noM-bM-^@M-^].
910
911 TrustedUserCAKeys
912 Specifies a file containing public keys of certificate
913 authorities that are trusted to sign user certificates for
914 authentication, or M-bM-^@M-^\noneM-bM-^@M-^] to not use one. Keys are listed one
915 per line; empty lines and comments starting with M-bM-^@M-^X#M-bM-^@M-^Y are allowed.
916 If a certificate is presented for authentication and has its
917 signing CA key listed in this file, then it may be used for
918 authentication for any user listed in the certificate's
919 principals list. Note that certificates that lack a list of
920 principals will not be permitted for authentication using
921 TrustedUserCAKeys. For more details on certificates, see the
922 CERTIFICATES section in ssh-keygen(1).
923
924 UseDNS Specifies whether sshd(8) should look up the remote host name,
925 and to check that the resolved host name for the remote IP
926 address maps back to the very same IP address.
927
928 If this option is set to M-bM-^@M-^\noM-bM-^@M-^] (the default) then only addresses
929 and not host names may be used in ~/.ssh/known_hosts from and
930 sshd_config(5) Match Host directives.
931
932 UseLogin
933 Specifies whether login(1) is used for interactive login
934 sessions. The default is M-bM-^@M-^\noM-bM-^@M-^]. Note that login(1) is never used
935 for remote command execution. Note also, that if this is
936 enabled, X11Forwarding will be disabled because login(1) does not
937 know how to handle xauth(1) cookies. If UsePrivilegeSeparation
938 is specified, it will be disabled after authentication.
939
940 UsePAM Enables the Pluggable Authentication Module interface. If set to
941 M-bM-^@M-^\yesM-bM-^@M-^] this will enable PAM authentication using
942 ChallengeResponseAuthentication and PasswordAuthentication in
943 addition to PAM account and session module processing for all
944 authentication types.
945
946 Because PAM challenge-response authentication usually serves an
947 equivalent role to password authentication, you should disable
948 either PasswordAuthentication or ChallengeResponseAuthentication.
949
950 If UsePAM is enabled, you will not be able to run sshd(8) as a
951 non-root user. The default is M-bM-^@M-^\noM-bM-^@M-^].
952
953 UsePrivilegeSeparation
954 Specifies whether sshd(8) separates privileges by creating an
955 unprivileged child process to deal with incoming network traffic.
956 After successful authentication, another process will be created
957 that has the privilege of the authenticated user. The goal of
958 privilege separation is to prevent privilege escalation by
959 containing any corruption within the unprivileged processes. The
960 default is M-bM-^@M-^\yesM-bM-^@M-^]. If UsePrivilegeSeparation is set to M-bM-^@M-^\sandboxM-bM-^@M-^]
961 then the pre-authentication unprivileged process is subject to
962 additional restrictions.
963
964 VersionAddendum
965 Optionally specifies additional text to append to the SSH
966 protocol banner sent by the server upon connection. The default
967 is M-bM-^@M-^\noneM-bM-^@M-^].
968
969 X11DisplayOffset
970 Specifies the first display number available for sshd(8)'s X11
971 forwarding. This prevents sshd from interfering with real X11
972 servers. The default is 10.
973
974 X11Forwarding
975 Specifies whether X11 forwarding is permitted. The argument must
976 be M-bM-^@M-^\yesM-bM-^@M-^] or M-bM-^@M-^\noM-bM-^@M-^]. The default is M-bM-^@M-^\noM-bM-^@M-^].
977
978 When X11 forwarding is enabled, there may be additional exposure
979 to the server and to client displays if the sshd(8) proxy display
980 is configured to listen on the wildcard address (see
981 X11UseLocalhost below), though this is not the default.
982 Additionally, the authentication spoofing and authentication data
983 verification and substitution occur on the client side. The
984 security risk of using X11 forwarding is that the client's X11
985 display server may be exposed to attack when the SSH client
986 requests forwarding (see the warnings for ForwardX11 in
987 ssh_config(5)). A system administrator may have a stance in
988 which they want to protect clients that may expose themselves to
989 attack by unwittingly requesting X11 forwarding, which can
990 warrant a M-bM-^@M-^\noM-bM-^@M-^] setting.
991
992 Note that disabling X11 forwarding does not prevent users from
993 forwarding X11 traffic, as users can always install their own
994 forwarders. X11 forwarding is automatically disabled if UseLogin
995 is enabled.
996
997 X11UseLocalhost
998 Specifies whether sshd(8) should bind the X11 forwarding server
999 to the loopback address or to the wildcard address. By default,
1000 sshd binds the forwarding server to the loopback address and sets
1001 the hostname part of the DISPLAY environment variable to
1002 M-bM-^@M-^\localhostM-bM-^@M-^]. This prevents remote hosts from connecting to the
1003 proxy display. However, some older X11 clients may not function
1004 with this configuration. X11UseLocalhost may be set to M-bM-^@M-^\noM-bM-^@M-^] to
1005 specify that the forwarding server should be bound to the
1006 wildcard address. The argument must be M-bM-^@M-^\yesM-bM-^@M-^] or M-bM-^@M-^\noM-bM-^@M-^]. The
1007 default is M-bM-^@M-^\yesM-bM-^@M-^].
1008
1009 XAuthLocation
1010 Specifies the full pathname of the xauth(1) program, or M-bM-^@M-^\noneM-bM-^@M-^] to
1011 not use one. The default is /usr/X11R6/bin/xauth.
1012
1013TIME FORMATS
1014 sshd(8) command-line arguments and configuration file options that
1015 specify time may be expressed using a sequence of the form:
1016 time[qualifier], where time is a positive integer value and qualifier is
1017 one of the following:
1018
1019 M-bM-^_M-(noneM-bM-^_M-) seconds
1020 s | S seconds
1021 m | M minutes
1022 h | H hours
1023 d | D days
1024 w | W weeks
1025
1026 Each member of the sequence is added together to calculate the total time
1027 value.
1028
1029 Time format examples:
1030
1031 600 600 seconds (10 minutes)
1032 10m 10 minutes
1033 1h30m 1 hour 30 minutes (90 minutes)
1034
1035FILES
1036 /etc/ssh/sshd_config
1037 Contains configuration data for sshd(8). This file should be
1038 writable by root only, but it is recommended (though not
1039 necessary) that it be world-readable.
1040
1041SEE ALSO
1042 sshd(8)
1043
1044AUTHORS
1045 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
1046 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
1047 de Raadt and Dug Song removed many bugs, re-added newer features and
1048 created OpenSSH. Markus Friedl contributed the support for SSH protocol
1049 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
1050 for privilege separation.
1051
1052OpenBSD 5.8 August 6, 2015 OpenBSD 5.8
generated by cgit v1.2.3 (git 2.39.1) at 2024-06-26 18:45:45 +0000