1SSHD_CONFIG(5) BSD File Formats Manual SSHD_CONFIG(5)
2
4 sshd_config — OpenSSH SSH daemon configuration file
5
7 /etc/ssh/sshd_config
8
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-argu‐
12 ment pairs, one per line. Lines starting with ‘#’ and empty lines are
13 interpreted as comments. Arguments may optionally be enclosed in double
14 quotes (") in order to represent arguments containing spaces.
15 The possible keywords and their meanings are as follows (note that key‐
17 words are case-insensitive and arguments are case-sensitive):
18 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 envi‐
23 ronment passing is only supported for protocol 2, and that the
24 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 ‘*’ and ‘?’. 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 AddressFamily
35 Specifies which address family should be used by sshd(8). Valid
36 arguments are “any”, “inet” (use IPv4 only), or “inet6” (use IPv6
37 only). The default is “any”.
38 AllowAgentForwarding
40 Specifies whether ssh-agent(1) forwarding is permitted. The
41 default is “yes”. 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 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. All of the specified user and group tests must suc‐
54 ceed, before user is allowed to log in.
55 See PATTERNS in ssh_config(5) for more information on patterns.
57 AllowTcpForwarding
59 Specifies whether TCP forwarding is permitted. The default is
60 “yes”. Note that disabling TCP forwarding does not improve secu‐
61 rity unless users are also denied shell access, as they can
62 always install their own forwarders.
63 AllowUsers
65 This keyword can be followed by a list of user name patterns,
66 separated by spaces. If specified, login is allowed only for
67 user names that match one of the patterns. Only user names are
68 valid; a numerical user ID is not recognized. By default, login
69 is allowed for all users. If the pattern takes the form
70 USER@HOST then USER and HOST are separately checked, restricting
71 logins to particular users from particular hosts. The allow/deny
72 directives are processed in the following order: DenyUsers,
73 AllowUsers, DenyGroups, and finally AllowGroups. All of the
74 specified user and group tests must succeed, before user is
75 allowed to log in.
76 See PATTERNS in ssh_config(5) for more information on patterns.
78 AuthorizedKeysFile
80 Specifies the file that contains the public keys that can be used
81 for user authentication. AuthorizedKeysFile may contain tokens
82 of the form %T which are substituted during connection setup.
83 The following tokens are defined: %% is replaced by a literal
84 '%', %h is replaced by the home directory of the user being
85 authenticated, and %u is replaced by the username of that user.
86 After expansion, AuthorizedKeysFile is taken to be an absolute
87 path or one relative to the user's home directory. The default
88 is “.ssh/authorized_keys”.
89 AuthorizedPrincipalsFile
91 Specifies a file that lists principal names that are accepted for
92 certificate authentication. When using certificates signed by a
93 key listed in TrustedUserCAKeys, this file lists names, one of
94 which must appear in the certificate for it to be accepted for
95 authentication. Names are listed one per line; empty lines and
96 comments starting with ‘#’ are ignored.
97 AuthorizedPrincipalsFile may contain tokens of the form %T which
99 are substituted during connection setup. The following tokens
100 are defined: %% is replaced by a literal '%', %h is replaced by
101 the home directory of the user being authenticated, and %u is
102 replaced by the username of that user. After expansion,
103 AuthorizedPrincipalsFile is taken to be an absolute path or one
104 relative to the user's home directory.
105 The default is not to use a principals file – in this case, the
107 username of the user must appear in a certificate's principals
108 list for it to be accepted. Note that AuthorizedPrincipalsFile
109 is only used when authentication proceeds using a CA listed in
110 TrustedUserCAKeys and is not consulted for certification authori‐
111 ties trusted via ~/.ssh/authorized_keys, though the principals=
112 key option offers a similar facility (see sshd(8) for details).
113 Banner The contents of the specified file are sent to the remote user
115 before authentication is allowed. If the argument is “none” then
116 no banner is displayed. This option is only available for proto‐
117 col version 2. By default, no banner is displayed.
118 ChallengeResponseAuthentication
120 Specifies whether challenge-response authentication is allowed
121 (e.g. via PAM or though authentication styles supported in
122 login.conf(5)) The default is “yes”.
123 ChrootDirectory
125 Specifies a path to chroot(2) to after authentication. This
126 path, and all its components, must be root-owned directories that
127 are not writable by any other user or group. After the chroot,
128 sshd(8) changes the working directory to the user's home direc‐
129 tory.
130 The path may contain the following tokens that are expanded at
132 runtime once the connecting user has been authenticated: %% is
133 replaced by a literal '%', %h is replaced by the home directory
134 of the user being authenticated, and %u is replaced by the user‐
135 name of that user.
136 The ChrootDirectory must contain the necessary files and directo‐
138 ries to support the user's session. For an interactive session
139 this requires at least a shell, typically sh(1), and basic /dev
140 nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4),
141 arandom(4) and tty(4) devices. For file transfer sessions using
142 “sftp”, no additional configuration of the environment is neces‐
143 sary if the in-process sftp server is used, though sessions which
144 use logging do require /dev/log inside the chroot directory (see
145 sftp-server(8) for details).
146 The default is not to chroot(2).
148 Ciphers
150 Specifies the ciphers allowed for protocol version 2. Multiple
151 ciphers must be comma-separated. The supported ciphers are
152 “3des-cbc”, “aes128-cbc”, “aes192-cbc”, “aes256-cbc”,
153 “aes128-ctr”, “aes192-ctr”, “aes256-ctr”, “arcfour128”,
154 “arcfour256”, “arcfour”, “blowfish-cbc”,
155 “rijndael-cbc@lysator.liu.se”, and “cast128-cbc”. The default
156 is:
157 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
159 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
160 aes256-cbc,arcfour,rijndael-cbc@lysator.liu.se
161 ClientAliveCountMax
163 Sets the number of client alive messages (see below) which may be
164 sent without sshd(8) receiving any messages back from the client.
165 If this threshold is reached while client alive messages are
166 being sent, sshd will disconnect the client, terminating the ses‐
167 sion. It is important to note that the use of client alive mes‐
168 sages is very different from TCPKeepAlive (below). The client
169 alive messages are sent through the encrypted channel and there‐
170 fore will not be spoofable. The TCP keepalive option enabled by
171 TCPKeepAlive is spoofable. The client alive mechanism is valu‐
172 able when the client or server depend on knowing when a connec‐
173 tion has become inactive.
174 The default value is 3. If ClientAliveInterval (see below) is
176 set to 15, and ClientAliveCountMax is left at the default, unre‐
177 sponsive SSH clients will be disconnected after approximately 45
178 seconds. This option applies to protocol version 2 only.
179 ClientAliveInterval
181 Sets a timeout interval in seconds after which if no data has
182 been received from the client, sshd(8) will send a message
183 through the encrypted channel to request a response from the
184 client. The default is 0, indicating that these messages will
185 not be sent to the client. This option applies to protocol ver‐
186 sion 2 only.
187 Compression
189 Specifies whether compression is allowed, or delayed until the
190 user has authenticated successfully. The argument must be “yes”,
191 “delayed”, or “no”. The default is “delayed”.
192 DenyGroups
194 This keyword can be followed by a list of group name patterns,
195 separated by spaces. Login is disallowed for users whose primary
196 group or supplementary group list matches one of the patterns.
197 Only group names are valid; a numerical group ID is not recog‐
198 nized. By default, login is allowed for all groups. The
199 allow/deny directives are processed in the following order:
200 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups. All
201 of the specified user and group tests must succeed, before user
202 is allowed to log in.
203 See PATTERNS in ssh_config(5) for more information on patterns.
205 DenyUsers
207 This keyword can be followed by a list of user name patterns,
208 separated by spaces. Login is disallowed for user names that
209 match one of the patterns. Only user names are valid; a numeri‐
210 cal user ID is not recognized. By default, login is allowed for
211 all users. If the pattern takes the form USER@HOST then USER and
212 HOST are separately checked, restricting logins to particular
213 users from particular hosts. The allow/deny directives are pro‐
214 cessed in the following order: DenyUsers, AllowUsers, DenyGroups,
215 and finally AllowGroups. All of the specified user and group
216 tests must succeed, before user is allowed to log in.
217 See PATTERNS in ssh_config(5) for more information on patterns.
219 ForceCommand
221 Forces the execution of the command specified by ForceCommand,
222 ignoring any command supplied by the client and ~/.ssh/rc if
223 present. The command is invoked by using the user's login shell
224 with the -c option. This applies to shell, command, or subsystem
225 execution. It is most useful inside a Match block. The command
226 originally supplied by the client is available in the
227 SSH_ORIGINAL_COMMAND environment variable. Specifying a command
228 of “internal-sftp” will force the use of an in-process sftp
229 server that requires no support files when used with
230 ChrootDirectory.
231 GatewayPorts
233 Specifies whether remote hosts are allowed to connect to ports
234 forwarded for the client. By default, sshd(8) binds remote port
235 forwardings to the loopback address. This prevents other remote
236 hosts from connecting to forwarded ports. GatewayPorts can be
237 used to specify that sshd should allow remote port forwardings to
238 bind to non-loopback addresses, thus allowing other hosts to con‐
239 nect. The argument may be “no” to force remote port forwardings
240 to be available to the local host only, “yes” to force remote
241 port forwardings to bind to the wildcard address, or
242 “clientspecified” to allow the client to select the address to
243 which the forwarding is bound. The default is “no”.
244 GSSAPIAuthentication
246 Specifies whether user authentication based on GSSAPI is allowed.
247 The default is “no”. Note that this option applies to protocol
248 version 2 only.
249 GSSAPIKeyExchange
251 Specifies whether key exchange based on GSSAPI is allowed. GSSAPI
252 key exchange doesn't rely on ssh keys to verify host identity.
253 The default is “no”. Note that this option applies to protocol
254 version 2 only.
255 GSSAPICleanupCredentials
257 Specifies whether to automatically destroy the user's credentials
258 cache on logout. The default is “yes”. Note that this option
259 applies to protocol version 2 only.
260 GSSAPIStrictAcceptorCheck
262 Determines whether to be strict about the identity of the GSSAPI
263 acceptor a client authenticates against. If “yes” then the client
264 must authenticate against the host service on the current host‐
265 name. If “no” then the client may authenticate against any ser‐
266 vice key stored in the machine's default store. This facility is
267 provided to assist with operation on multi homed machines. The
268 default is “yes”. Note that this option applies only to protocol
269 version 2 GSSAPI connections, and setting it to “no” may only
270 work with recent Kerberos GSSAPI libraries.
271 GSSAPIStoreCredentialsOnRekey
273 Controls whether the user's GSSAPI credentials should be updated
274 following a successful connection rekeying. This option can be
275 used to accepted renewed or updated credentials from a compatible
276 client. The default is “no”.
277 GSSAPIKexAlgorithms
279 The list of key exchange algorithms that are accepted by GSSAPI
280 key exchange. Possible values are
281 gss-group1-sha1-,
283 gss-group14-sha1-,
284 gss-gex-sha1-
285 The default is
287 “gss-group1-sha1-,gss-group14-sha1-,gss-gex-sha1-”. This option
288 only applies to protocol version 2 connections using GSSAPI.
289 HostbasedAuthentication
291 Specifies whether rhosts or /etc/hosts.equiv authentication
292 together with successful public key client host authentication is
293 allowed (host-based authentication). This option is similar to
294 RhostsRSAAuthentication and applies to protocol version 2 only.
295 The default is “no”.
296 HostbasedUsesNameFromPacketOnly
298 Specifies whether or not the server will attempt to perform a
299 reverse name lookup when matching the name in the ~/.shosts,
300 ~/.rhosts, and /etc/hosts.equiv files during
301 HostbasedAuthentication. A setting of “yes” means that sshd(8)
302 uses the name supplied by the client rather than attempting to
303 resolve the name from the TCP connection itself. The default is
304 “no”.
305 HostCertificate
307 Specifies a file containing a public host certificate. The cer‐
308 tificate's public key must match a private host key already spec‐
309 ified by HostKey. The default behaviour of sshd(8) is not to
310 load any certificates.
311 HostKey
313 Specifies a file containing a private host key used by SSH. The
314 default is /etc/ssh/ssh_host_key for protocol version 1, and
315 /etc/ssh/ssh_host_dsa_key and /etc/ssh/ssh_host_rsa_key for pro‐
316 tocol version 2. Note that sshd(8) will refuse to use a file if
317 it is group/world-accessible. It is possible to have multiple
318 host key files. “rsa1” keys are used for version 1 and “dsa”,
319 “ecdsa” or “rsa” are used for version 2 of the SSH protocol.
320 IgnoreRhosts
322 Specifies that .rhosts and .shosts files will not be used in
323 RhostsRSAAuthentication or HostbasedAuthentication.
324 /etc/hosts.equiv and /etc/ssh/shosts.equiv are still used. The
326 default is “yes”.
327 IgnoreUserKnownHosts
329 Specifies whether sshd(8) should ignore the user's
330 ~/.ssh/known_hosts during RhostsRSAAuthentication or
331 HostbasedAuthentication. The default is “no”.
332 KbdInteractiveAuthentication
334 Specifies whether to allow keyboard-interactive authentication.
335 The argument to this keyword must be “yes” or “no”. The default
336 is to use whatever value ChallengeResponseAuthentication is set
337 to (by default “yes”).
338 KerberosAuthentication
340 Specifies whether the password provided by the user for
341 PasswordAuthentication will be validated through the Kerberos
342 KDC. To use this option, the server needs a Kerberos servtab
343 which allows the verification of the KDC's identity. The default
344 is “no”.
345 KerberosGetAFSToken
347 If AFS is active and the user has a Kerberos 5 TGT, attempt to
348 acquire an AFS token before accessing the user's home directory.
349 The default is “no”.
350 KerberosOrLocalPasswd
352 If password authentication through Kerberos fails then the pass‐
353 word will be validated via any additional local mechanism such as
354 /etc/passwd. The default is “yes”.
355 KerberosTicketCleanup
357 Specifies whether to automatically destroy the user's ticket
358 cache file on logout. The default is “yes”.
359 KerberosUseKuserok
361 Specifies whether to look at .k5login file for user's aliases.
362 The default is “yes”.
363 KexAlgorithms
365 Specifies the available KEX (Key Exchange) algorithms. Multiple
366 algorithms must be comma-separated. The default is
367 “diffie-hellman-group-exchange-sha256”,
368 “diffie-hellman-group-exchange-sha1”,
369 “diffie-hellman-group14-sha1”, “diffie-hellman-group1-sha1”.
370 KeyRegenerationInterval
372 In protocol version 1, the ephemeral server key is automatically
373 regenerated after this many seconds (if it has been used). The
374 purpose of regeneration is to prevent decrypting captured ses‐
375 sions by later breaking into the machine and stealing the keys.
376 The key is never stored anywhere. If the value is 0, the key is
377 never regenerated. The default is 3600 (seconds).
378 ListenAddress
380 Specifies the local addresses sshd(8) should listen on. The fol‐
381 lowing forms may be used:
382 ListenAddress host|IPv4_addr|IPv6_addr
384 ListenAddress host|IPv4_addr:port
385 ListenAddress [host|IPv6_addr]:port
386 If port is not specified, sshd will listen on the address and all
388 prior Port options specified. The default is to listen on all
389 local addresses. Multiple ListenAddress options are permitted.
390 Additionally, any Port options must precede this option for non-
391 port qualified addresses.
392 LoginGraceTime
394 The server disconnects after this time if the user has not suc‐
395 cessfully logged in. If the value is 0, there is no time limit.
396 The default is 120 seconds.
397 LogLevel
399 Gives the verbosity level that is used when logging messages from
400 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO,
401 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
402 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
403 higher levels of debugging output. Logging with a DEBUG level
404 violates the privacy of users and is not recommended.
405 MACs Specifies the available MAC (message authentication code) algo‐
407 rithms. The MAC algorithm is used in protocol version 2 for data
408 integrity protection. Multiple algorithms must be comma-sepa‐
409 rated. The default is:
410 hmac-md5,hmac-sha1,umac-64@openssh.com,
412 hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
413 hmac-sha2-256,hmac-sha2-512,hmac-ripemd160@openssh.com
414 Match Introduces a conditional block. If all of the criteria on the
416 Match line are satisfied, the keywords on the following lines
417 override those set in the global section of the config file,
418 until either another Match line or the end of the file. If a
419 keyword appears in multiple Match blocks that are satisfied, only
420 the first instance of the keyword is applied.
421 The arguments to Match are one or more criteria-pattern pairs.
423 The available criteria are User, Group, Host, LocalAddress,
424 LocalPort, and Address. The match patterns may consist of single
425 entries or comma-separated lists and may use the wildcard and
426 negation operators described in the PATTERNS section of
427 ssh_config(5).
428 The patterns in an Address criteria may additionally contain
430 addresses to match in CIDR address/masklen format, e.g.
431 “192.0.2.0/24” or “3ffe:ffff::/32”. Note that the mask length
432 provided must be consistent with the address - it is an error to
433 specify a mask length that is too long for the address or one
434 with bits set in this host portion of the address. For example,
435 “192.0.2.0/33” and “192.0.2.0/8” respectively.
436 Only a subset of keywords may be used on the lines following a
438 Match keyword. Available keywords are AllowAgentForwarding,
439 AllowTcpForwarding, Banner, ChrootDirectory, ForceCommand,
440 GatewayPorts, GSSAPIAuthentication, HostbasedAuthentication,
441 KbdInteractiveAuthentication, KerberosAuthentication,
442 KerberosUseKuserok, MaxAuthTries, MaxSessions,
443 PubkeyAuthentication, AuthorizedKeysCommand,
444 AuthorizedKeysCommandRunAs, PasswordAuthentication,
445 PermitEmptyPasswords, PermitOpen, PermitRootLogin,
446 RequiredAuthentications1, RequiredAuthentications2,
447 RhostsRSAAuthentication, RSAAuthentication, X11DisplayOffset,
448 X11Forwarding and X11UseLocalHost.
449 MaxAuthTries
451 Specifies the maximum number of authentication attempts permitted
452 per connection. Once the number of failures reaches half this
453 value, additional failures are logged. The default is 6.
454 MaxSessions
456 Specifies the maximum number of open sessions permitted per net‐
457 work connection. The default is 10.
458 MaxStartups
460 Specifies the maximum number of concurrent unauthenticated con‐
461 nections to the SSH daemon. Additional connections will be
462 dropped until authentication succeeds or the LoginGraceTime
463 expires for a connection. The default is 10:30:100.
464 Alternatively, random early drop can be enabled by specifying the
466 three colon separated values “start:rate:full” (e.g. "10:30:60").
467 sshd(8) will refuse connection attempts with a probability of
468 “rate/100” (30%) if there are currently “start” (10) unauthenti‐
469 cated connections. The probability increases linearly and all
470 connection attempts are refused if the number of unauthenticated
471 connections reaches “full” (60).
472 PasswordAuthentication
474 Specifies whether password authentication is allowed. The
475 default is “yes”.
476 PermitEmptyPasswords
478 When password authentication is allowed, it specifies whether the
479 server allows login to accounts with empty password strings. The
480 default is “no”.
481 PermitOpen
483 Specifies the destinations to which TCP port forwarding is per‐
484 mitted. The forwarding specification must be one of the follow‐
485 ing forms:
486 PermitOpen host:port
488 PermitOpen IPv4_addr:port
489 PermitOpen [IPv6_addr]:port
490 Multiple forwards may be specified by separating them with white‐
492 space. An argument of “any” can be used to remove all restric‐
493 tions and permit any forwarding requests. By default all port
494 forwarding requests are permitted.
495 PermitRootLogin
497 Specifies whether root can log in using ssh(1). The argument
498 must be “yes”, “without-password”, “forced-commands-only”, or
499 “no”. The default is “yes”.
500 If this option is set to “without-password”, password authentica‐
502 tion is disabled for root.
503 If this option is set to “forced-commands-only”, root login with
505 public key authentication will be allowed, but only if the
506 command option has been specified (which may be useful for taking
507 remote backups even if root login is normally not allowed). All
508 other authentication methods are disabled for root.
509 If this option is set to “no”, root is not allowed to log in.
511 PermitTunnel
513 Specifies whether tun(4) device forwarding is allowed. The argu‐
514 ment must be “yes”, “point-to-point” (layer 3), “ethernet” (layer
515 2), or “no”. Specifying “yes” permits both “point-to-point” and
516 “ethernet”. The default is “no”.
517 PermitUserEnvironment
519 Specifies whether ~/.ssh/environment and environment= options in
520 ~/.ssh/authorized_keys are processed by sshd(8). The default is
521 “no”. Enabling environment processing may enable users to bypass
522 access restrictions in some configurations using mechanisms such
523 as LD_PRELOAD.
524 PidFile
526 Specifies the file that contains the process ID of the SSH dae‐
527 mon. The default is /var/run/sshd.pid.
528 Port Specifies the port number that sshd(8) listens on. The default
530 is 22. Multiple options of this type are permitted. See also
531 ListenAddress.
532 PrintLastLog
534 Specifies whether sshd(8) should print the date and time of the
535 last user login when a user logs in interactively. The default
536 is “yes”.
537 PrintMotd
539 Specifies whether sshd(8) should print /etc/motd when a user logs
540 in interactively. (On some systems it is also printed by the
541 shell, /etc/profile, or equivalent.) The default is “yes”.
542 Protocol
544 Specifies the protocol versions sshd(8) supports. The possible
545 values are ‘1’ and ‘2’. Multiple versions must be comma-sepa‐
546 rated. The default is “2,1”. Note that the order of the proto‐
547 col list does not indicate preference, because the client selects
548 among multiple protocol versions offered by the server. Specify‐
549 ing “2,1” is identical to “1,2”.
550 PubkeyAuthentication
552 Specifies whether public key authentication is allowed. The
553 default is “yes”. Note that this option applies to protocol ver‐
554 sion 2 only.
555 AuthorizedKeysCommand
557 Specifies a program to be used for lookup of the user's public
558 keys. The program will be invoked with its first argument the
559 name of the user being authorized, and should produce on standard
560 output AuthorizedKeys lines (see AUTHORIZED_KEYS in sshd(8)). By
561 default (or when set to the empty string) there is no Authorized‐
562 KeysCommand run. If the AuthorizedKeysCommand does not success‐
563 fully authorize the user, authorization falls through to the
564 AuthorizedKeysFile. Note that this option has an effect only
565 with PubkeyAuthentication turned on.
566 AuthorizedKeysCommandRunAs
568 Specifies the user under whose account the AuthorizedKeysCommand
569 is run. Empty string (the default value) means the user being
570 authorized is used.
571 RequiredAuthentications[12]
573 Specifies required methods of authentications that has to succeed
574 before authorizing the connection. (RequiredAuthentication1 for
575 Protocol version 1, and RequiredAuthentication2 for v2)
576 RequiredAuthentications1 method[,method...]
578 RequiredAuthentications2 method[,method...]
579 Example 1:
581 RequiredAuthentications2 password,hostbased
583 Example 2:
585 RequiredAuthentications2 publickey,password
587 Available methods:
589 password, keyboard-interactive, publickey, hostbased, gssapi-keyex, gssapi-with-mic
591 RevokedKeys
593 Specifies a list of revoked public keys. Keys listed in this
594 file will be refused for public key authentication. Note that if
595 this file is not readable, then public key authentication will be
596 refused for all users.
597 RhostsRSAAuthentication
599 Specifies whether rhosts or /etc/hosts.equiv authentication
600 together with successful RSA host authentication is allowed. The
601 default is “no”. This option applies to protocol version 1 only.
602 RSAAuthentication
604 Specifies whether pure RSA authentication is allowed. The
605 default is “yes”. This option applies to protocol version 1
606 only.
607 ServerKeyBits
609 Defines the number of bits in the ephemeral protocol version 1
610 server key. The minimum value is 512, and the default is 1024.
611 ShowPatchLevel
613 Specifies whether sshd will display the patch level of the binary
614 in the identification string. The patch level is set at compile-
615 time. The default is “no”. This option applies to protocol ver‐
616 sion 1 only.
617 StrictModes
619 Specifies whether sshd(8) should check file modes and ownership
620 of the user's files and home directory before accepting login.
621 This is normally desirable because novices sometimes accidentally
622 leave their directory or files world-writable. The default is
623 “yes”.
624 Subsystem
626 Configures an external subsystem (e.g. file transfer daemon).
627 Arguments should be a subsystem name and a command (with optional
628 arguments) to execute upon subsystem request.
629 The command sftp-server(8) implements the “sftp” file transfer
631 subsystem.
632 Alternately the name “internal-sftp” implements an in-process
634 “sftp” server. This may simplify configurations using
635 ChrootDirectory to force a different filesystem root on clients.
636 By default no subsystems are defined. Note that this option
638 applies to protocol version 2 only.
639 SyslogFacility
641 Gives the facility code that is used when logging messages from
642 sshd(8). The possible values are: DAEMON, USER, AUTH, AUTHPRIV,
643 LOCAL0, LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7.
644 The default is AUTH.
645 TCPKeepAlive
647 Specifies whether the system should send TCP keepalive messages
648 to the other side. If they are sent, death of the connection or
649 crash of one of the machines will be properly noticed. However,
650 this means that connections will die if the route is down tempo‐
651 rarily, and some people find it annoying. On the other hand, if
652 TCP keepalives are not sent, sessions may hang indefinitely on
653 the server, leaving “ghost” users and consuming server resources.
654 The default is “yes” (to send TCP keepalive messages), and the
656 server will notice if the network goes down or the client host
657 crashes. This avoids infinitely hanging sessions.
658 To disable TCP keepalive messages, the value should be set to
660 “no”.
661 TrustedUserCAKeys
663 Specifies a file containing public keys of certificate authori‐
664 ties that are trusted sign user certificates for authentication.
665 Keys are listed one per line, empty lines and comments starting
666 with ‘#’ are allowed. If a certificate is presented for authen‐
667 tication and has its signing CA key listed in this file, then it
668 may be used for authentication for any user listed in the cer‐
669 tificate's principals list. Note that certificates that lack a
670 list of principals will not be permitted for authentication using
671 TrustedUserCAKeys. For more details in certificates, please see
672 the CERTIFICATES section in ssh-keygen(1).
673 UseDNS Specifies whether sshd(8) should look up the remote host name and
675 check that the resolved host name for the remote IP address maps
676 back to the very same IP address. The default is “yes”.
677 UseLogin
679 Specifies whether login(1) is used for interactive login ses‐
680 sions. The default is “no”. Note that login(1) is never used
681 for remote command execution. Note also, that if this is
682 enabled, X11Forwarding will be disabled because login(1) does not
683 know how to handle xauth(1) cookies. If UsePrivilegeSeparation
684 is specified, it will be disabled after authentication.
685 UsePAM Enables the Pluggable Authentication Module interface. If set to
687 “yes” this will enable PAM authentication using
688 ChallengeResponseAuthentication and PasswordAuthentication in
689 addition to PAM account and session module processing for all
690 authentication types.
691 Because PAM challenge-response authentication usually serves an
693 equivalent role to password authentication, you should disable
694 either PasswordAuthentication or ChallengeResponseAuthentication.
695 If UsePAM is enabled, you will not be able to run sshd(8) as a
697 non-root user. The default is “no”.
698 UsePrivilegeSeparation
700 Specifies whether sshd(8) separates privileges by creating an
701 unprivileged child process to deal with incoming network traffic.
702 After successful authentication, another process will be created
703 that has the privilege of the authenticated user. The goal of
704 privilege separation is to prevent privilege escalation by con‐
705 taining any corruption within the unprivileged processes. The
706 default is “yes”.
707 X11DisplayOffset
709 Specifies the first display number available for sshd(8)'s X11
710 forwarding. This prevents sshd from interfering with real X11
711 servers. The default is 10.
712 X11Forwarding
714 Specifies whether X11 forwarding is permitted. The argument must
715 be “yes” or “no”. The default is “no”.
716 When X11 forwarding is enabled, there may be additional exposure
718 to the server and to client displays if the sshd(8) proxy display
719 is configured to listen on the wildcard address (see
720 X11UseLocalhost below), though this is not the default. Addi‐
721 tionally, the authentication spoofing and authentication data
722 verification and substitution occur on the client side. The
723 security risk of using X11 forwarding is that the client's X11
724 display server may be exposed to attack when the SSH client
725 requests forwarding (see the warnings for ForwardX11 in
726 ssh_config(5)). A system administrator may have a stance in
727 which they want to protect clients that may expose themselves to
728 attack by unwittingly requesting X11 forwarding, which can war‐
729 rant a “no” setting.
730 Note that disabling X11 forwarding does not prevent users from
732 forwarding X11 traffic, as users can always install their own
733 forwarders. X11 forwarding is automatically disabled if UseLogin
734 is enabled.
735 X11UseLocalhost
737 Specifies whether sshd(8) should bind the X11 forwarding server
738 to the loopback address or to the wildcard address. By default,
739 sshd binds the forwarding server to the loopback address and sets
740 the hostname part of the DISPLAY environment variable to
741 “localhost”. This prevents remote hosts from connecting to the
742 proxy display. However, some older X11 clients may not function
743 with this configuration. X11UseLocalhost may be set to “no” to
744 specify that the forwarding server should be bound to the wild‐
745 card address. The argument must be “yes” or “no”. The default
746 is “yes”.
747 XAuthLocation
749 Specifies the full pathname of the xauth(1) program. The default
750 is /usr/bin/xauth.
751
753 sshd(8) command-line arguments and configuration file options that spec‐
754 ify time may be expressed using a sequence of the form: time[qualifier],
755 where time is a positive integer value and qualifier is one of the fol‐
756 lowing:
757 ⟨none⟩ seconds
759 s | S seconds
760 m | M minutes
761 h | H hours
762 d | D days
763 w | W weeks
764 Each member of the sequence is added together to calculate the total time
766 value.
767 Time format examples:
769 600 600 seconds (10 minutes)
771 10m 10 minutes
772 1h30m 1 hour 30 minutes (90 minutes)
773
775 /etc/ssh/sshd_config
776 Contains configuration data for sshd(8). This file should be
777 writable by root only, but it is recommended (though not neces‐
778 sary) that it be world-readable.
779
781 sshd(8)
782
784 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
785 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
786 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
787 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
788 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
789 for privilege separation.
790BSD June 21, 2019 BSD