1 SSHD_CONFIG(5) OpenBSD Programmer's Manual SSHD_CONFIG(5)
4 sshd_config - OpenSSH SSH daemon configuration file
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 `#' and empty lines
13 are interpreted as comments. Arguments may optionally be enclosed in
14 double quotes (") in order to represent arguments containing spaces.
16 The possible keywords and their meanings are as follows (note that
17 keywords are case-insensitive and arguments are case-sensitive):
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. Variables
24 are specified by name, which may contain the wildcard characters
25 `*' and `?'. Multiple environment variables may be separated by
26 whitespace or spread across multiple AcceptEnv directives. Be
27 warned that some environment variables could be used to bypass
28 restricted user environments. For this reason, care should be
29 taken in the use of this directive. The default is not to accept
30 any environment variables.
33 Specifies which address family should be used by sshd(8). Valid
34 arguments are ``any'', ``inet'' (use IPv4 only), or ``inet6''
35 (use IPv6 only). The default is ``any''.
38 Specifies whether ssh-agent(1) forwarding is permitted. The
39 default is ``yes''. Note that disabling agent forwarding does
40 not improve security unless users are also denied shell access,
41 as they can always install their own forwarders.
44 This keyword can be followed by a list of group name patterns,
45 separated by spaces. If specified, login is allowed only for
46 users whose primary group or supplementary group list matches one
47 of the patterns. Only group names are valid; a numerical group
48 ID is not recognized. By default, login is allowed for all
49 groups. The allow/deny directives are processed in the following
50 order: DenyUsers, AllowUsers, DenyGroups, and finally
53 See PATTERNS in ssh_config(5) for more information on patterns.
56 Specifies whether TCP forwarding is permitted. The default is
57 ``yes''. Note that disabling TCP forwarding does not improve
58 security unless users are also denied shell access, as they can
59 always install their own forwarders.
62 This keyword can be followed by a list of user name patterns,
63 separated by spaces. If specified, login is allowed only for
64 user names that match one of the patterns. Only user names are
65 valid; a numerical user ID is not recognized. By default, login
66 is allowed for all users. If the pattern takes the form
67 USER@HOST then USER and HOST are separately checked, restricting
68 logins to particular users from particular hosts. The allow/deny
69 directives are processed in the following order: DenyUsers,
70 AllowUsers, DenyGroups, and finally AllowGroups.
72 See PATTERNS in ssh_config(5) for more information on patterns.
75 Specifies the file that contains the public keys that can be used
76 for user authentication. The format is described in the
77 AUTHORIZED_KEYS FILE FORMAT section of sshd(8).
78 AuthorizedKeysFile may contain tokens of the form %T which are
79 substituted during connection setup. The following tokens are
80 defined: %% is replaced by a literal '%', %h is replaced by the
81 home directory of the user being authenticated, and %u is
82 replaced by the username of that user. After expansion,
83 AuthorizedKeysFile is taken to be an absolute path or one
84 relative to the user's home directory. Multiple files may be
85 listed, separated by whitespace. The default is
86 ``.ssh/authorized_keys .ssh/authorized_keys2''.
88 AuthorizedPrincipalsFile
89 Specifies a file that lists principal names that are accepted for
90 certificate authentication. When using certificates signed by a
91 key listed in TrustedUserCAKeys, this file lists names, one of
92 which must appear in the certificate for it to be accepted for
93 authentication. Names are listed one per line preceded by key
94 options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)).
95 Empty lines and comments starting with `#' are ignored.
97 AuthorizedPrincipalsFile may contain tokens of the form %T which
98 are substituted during connection setup. The following tokens
99 are defined: %% is replaced by a literal '%', %h is replaced by
100 the home directory of the user being authenticated, and %u is
101 replaced by the username of that user. After expansion,
102 AuthorizedPrincipalsFile is taken to be an absolute path or one
103 relative to the user's home directory.
105 The default is not to use a principals file - in this case, the
106 username of the user must appear in a certificate's principals
107 list for it to be accepted. Note that AuthorizedPrincipalsFile
108 is only used when authentication proceeds using a CA listed in
109 TrustedUserCAKeys and is not consulted for certification
110 authorities trusted via ~/.ssh/authorized_keys, though the
111 principals= key option offers a similar facility (see sshd(8) for
114 Banner The contents of the specified file are sent to the remote user
115 before authentication is allowed. If the argument is ``none''
116 then no banner is displayed. This option is only available for
117 protocol version 2. By default, no banner is displayed.
119 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''.
125 Specifies the pathname of a directory to chroot(2) to after
126 authentication. All components of the pathname must be root-
127 owned directories that are not writable by any other user or
128 group. After the chroot, sshd(8) changes the working directory
129 to the user's home directory.
131 The pathname may contain the following tokens that are expanded
132 at 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
135 username of that user.
137 The ChrootDirectory must contain the necessary files and
138 directories to support the user's session. For an interactive
139 session this requires at least a shell, typically sh(1), and
140 basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4),
141 stderr(4), arandom(4) and tty(4) devices. For file transfer
142 sessions using ``sftp'', no additional configuration of the
143 environment is necessary if the in-process sftp server is used,
144 though sessions which use logging do require /dev/log inside the
145 chroot directory (see sftp-server(8) for details).
147 The default is not to chroot(2).
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'', and
155 ``cast128-cbc''. The default is:
157 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128,
158 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc,
162 Sets the number of client alive messages (see below) which may be
163 sent without sshd(8) receiving any messages back from the client.
164 If this threshold is reached while client alive messages are
165 being sent, sshd will disconnect the client, terminating the
166 session. It is important to note that the use of client alive
167 messages is very different from TCPKeepAlive (below). The client
168 alive messages are sent through the encrypted channel and
169 therefore will not be spoofable. The TCP keepalive option
170 enabled by TCPKeepAlive is spoofable. The client alive mechanism
171 is valuable when the client or server depend on knowing when a
172 connection has become inactive.
174 The default value is 3. If ClientAliveInterval (see below) is
175 set to 15, and ClientAliveCountMax is left at the default,
176 unresponsive SSH clients will be disconnected after approximately
177 45 seconds. This option applies to protocol version 2 only.
180 Sets a timeout interval in seconds after which if no data has
181 been received from the client, sshd(8) will send a message
182 through the encrypted channel to request a response from the
183 client. The default is 0, indicating that these messages will
184 not be sent to the client. This option applies to protocol
188 Specifies whether compression is allowed, or delayed until the
189 user has authenticated successfully. The argument must be
190 ``yes'', ``delayed'', or ``no''. The default is ``delayed''.
193 This keyword can be followed by a list of group name patterns,
194 separated by spaces. Login is disallowed for users whose primary
195 group or supplementary group list matches one of the patterns.
196 Only group names are valid; a numerical group ID is not
197 recognized. By default, login is allowed for all groups. The
198 allow/deny directives are processed in the following order:
199 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups.
201 See PATTERNS in ssh_config(5) for more information on patterns.
204 This keyword can be followed by a list of user name patterns,
205 separated by spaces. Login is disallowed for user names that
206 match one of the patterns. Only user names are valid; a
207 numerical user ID is not recognized. By default, login is
208 allowed for all users. If the pattern takes the form USER@HOST
209 then USER and HOST are separately checked, restricting logins to
210 particular users from particular hosts. The allow/deny
211 directives are processed in the following order: DenyUsers,
212 AllowUsers, DenyGroups, and finally AllowGroups.
214 See PATTERNS in ssh_config(5) for more information on patterns.
217 Forces the execution of the command specified by ForceCommand,
218 ignoring any command supplied by the client and ~/.ssh/rc if
219 present. The command is invoked by using the user's login shell
220 with the -c option. This applies to shell, command, or subsystem
221 execution. It is most useful inside a Match block. The command
222 originally supplied by the client is available in the
223 SSH_ORIGINAL_COMMAND environment variable. Specifying a command
224 of ``internal-sftp'' will force the use of an in-process sftp
225 server that requires no support files when used with
229 Specifies whether remote hosts are allowed to connect to ports
230 forwarded for the client. By default, sshd(8) binds remote port
231 forwardings to the loopback address. This prevents other remote
232 hosts from connecting to forwarded ports. GatewayPorts can be
233 used to specify that sshd should allow remote port forwardings to
234 bind to non-loopback addresses, thus allowing other hosts to
235 connect. The argument may be ``no'' to force remote port
236 forwardings to be available to the local host only, ``yes'' to
237 force remote port forwardings to bind to the wildcard address, or
238 ``clientspecified'' to allow the client to select the address to
239 which the forwarding is bound. The default is ``no''.
242 Specifies whether user authentication based on GSSAPI is allowed.
243 The default is ``no''. Note that this option applies to protocol
246 GSSAPICleanupCredentials
247 Specifies whether to automatically destroy the user's credentials
248 cache on logout. The default is ``yes''. Note that this option
249 applies to protocol version 2 only.
251 HostbasedAuthentication
252 Specifies whether rhosts or /etc/hosts.equiv authentication
253 together with successful public key client host authentication is
254 allowed (host-based authentication). This option is similar to
255 RhostsRSAAuthentication and applies to protocol version 2 only.
256 The default is ``no''.
258 HostbasedUsesNameFromPacketOnly
259 Specifies whether or not the server will attempt to perform a
260 reverse name lookup when matching the name in the ~/.shosts,
261 ~/.rhosts, and /etc/hosts.equiv files during
262 HostbasedAuthentication. A setting of ``yes'' means that sshd(8)
263 uses the name supplied by the client rather than attempting to
264 resolve the name from the TCP connection itself. The default is
268 Specifies a file containing a public host certificate. The
269 certificate's public key must match a private host key already
270 specified by HostKey. The default behaviour of sshd(8) is not to
271 load any certificates.
274 Specifies a file containing a private host key used by SSH. The
275 default is /etc/ssh/ssh_host_key for protocol version 1, and
276 /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key and
277 /etc/ssh/ssh_host_rsa_key for protocol version 2. Note that
278 sshd(8) will refuse to use a file if it is group/world-
279 accessible. It is possible to have multiple host key files.
280 ``rsa1'' keys are used for version 1 and ``dsa'', ``ecdsa'' or
281 ``rsa'' are used for version 2 of the SSH protocol.
284 Specifies that .rhosts and .shosts files will not be used in
285 RhostsRSAAuthentication or HostbasedAuthentication.
287 /etc/hosts.equiv and /etc/shosts.equiv are still used. The
291 Specifies whether sshd(8) should ignore the user's
292 ~/.ssh/known_hosts during RhostsRSAAuthentication or
293 HostbasedAuthentication. The default is ``no''.
295 IPQoS Specifies the IPv4 type-of-service or DSCP class for the
296 connection. Accepted values are ``af11'', ``af12'', ``af13'',
297 ``af14'', ``af22'', ``af23'', ``af31'', ``af32'', ``af33'',
298 ``af41'', ``af42'', ``af43'', ``cs0'', ``cs1'', ``cs2'', ``cs3'',
299 ``cs4'', ``cs5'', ``cs6'', ``cs7'', ``ef'', ``lowdelay'',
300 ``throughput'', ``reliability'', or a numeric value. This option
301 may take one or two arguments, separated by whitespace. If one
302 argument is specified, it is used as the packet class
303 unconditionally. If two values are specified, the first is
304 automatically selected for interactive sessions and the second
305 for non-interactive sessions. The default is ``lowdelay'' for
306 interactive sessions and ``throughput'' for non-interactive
309 KerberosAuthentication
310 Specifies whether the password provided by the user for
311 PasswordAuthentication will be validated through the Kerberos
312 KDC. To use this option, the server needs a Kerberos servtab
313 which allows the verification of the KDC's identity. The default
317 If AFS is active and the user has a Kerberos 5 TGT, attempt to
318 acquire an AFS token before accessing the user's home directory.
319 The default is ``no''.
321 KerberosOrLocalPasswd
322 If password authentication through Kerberos fails then the
323 password will be validated via any additional local mechanism
324 such as /etc/passwd. The default is ``yes''.
326 KerberosTicketCleanup
327 Specifies whether to automatically destroy the user's ticket
328 cache file on logout. The default is ``yes''.
331 Specifies the available KEX (Key Exchange) algorithms. Multiple
332 algorithms must be comma-separated. The default is
333 ``ecdh-sha2-nistp256'', ``ecdh-sha2-nistp384'',
334 ``ecdh-sha2-nistp521'', ``diffie-hellman-group-exchange-sha256'',
335 ``diffie-hellman-group-exchange-sha1'',
336 ``diffie-hellman-group14-sha1'', ``diffie-hellman-group1-sha1''.
338 KeyRegenerationInterval
339 In protocol version 1, the ephemeral server key is automatically
340 regenerated after this many seconds (if it has been used). The
341 purpose of regeneration is to prevent decrypting captured
342 sessions by later breaking into the machine and stealing the
343 keys. The key is never stored anywhere. If the value is 0, the
344 key is never regenerated. The default is 3600 (seconds).
347 Specifies the local addresses sshd(8) should listen on. The
348 following forms may be used:
350 ListenAddress host|IPv4_addr|IPv6_addr
351 ListenAddress host|IPv4_addr:port
352 ListenAddress [host|IPv6_addr]:port
354 If port is not specified, sshd will listen on the address and all
355 prior Port options specified. The default is to listen on all
356 local addresses. Multiple ListenAddress options are permitted.
357 Additionally, any Port options must precede this option for non-
358 port qualified addresses.
361 The server disconnects after this time if the user has not
362 successfully logged in. If the value is 0, there is no time
363 limit. The default is 120 seconds.
366 Gives the verbosity level that is used when logging messages from
367 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO,
368 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO.
369 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify
370 higher levels of debugging output. Logging with a DEBUG level
371 violates the privacy of users and is not recommended.
373 MACs Specifies the available MAC (message authentication code)
374 algorithms. The MAC algorithm is used in protocol version 2 for
375 data integrity protection. Multiple algorithms must be comma-
376 separated. The default is:
378 hmac-md5,hmac-sha1,umac-64@openssh.com,
379 hmac-ripemd160,hmac-sha1-96,hmac-md5-96,
380 hmac-sha2-256,hmac-sha256-96,hmac-sha2-512,
383 Match Introduces a conditional block. If all of the criteria on the
384 Match line are satisfied, the keywords on the following lines
385 override those set in the global section of the config file,
386 until either another Match line or the end of the file.
388 The arguments to Match are one or more criteria-pattern pairs.
389 The available criteria are User, Group, Host, and Address. The
390 match patterns may consist of single entries or comma-separated
391 lists and may use the wildcard and negation operators described
392 in the PATTERNS section of ssh_config(5).
394 The patterns in an Address criteria may additionally contain
395 addresses to match in CIDR address/masklen format, e.g.
396 ``192.0.2.0/24'' or ``3ffe:ffff::/32''. Note that the mask
397 length provided must be consistent with the address - it is an
398 error to specify a mask length that is too long for the address
399 or one with bits set in this host portion of the address. For
400 example, ``192.0.2.0/33'' and ``192.0.2.0/8'' respectively.
402 Only a subset of keywords may be used on the lines following a
403 Match keyword. Available keywords are AllowAgentForwarding,
404 AllowTcpForwarding, AuthorizedKeysFile, AuthorizedPrincipalsFile,
405 Banner, ChrootDirectory, ForceCommand, GatewayPorts,
406 GSSAPIAuthentication, HostbasedAuthentication,
407 HostbasedUsesNameFromPacketOnly, KbdInteractiveAuthentication,
408 KerberosAuthentication, MaxAuthTries, MaxSessions,
409 PasswordAuthentication, PermitEmptyPasswords, PermitOpen,
410 PermitRootLogin, PermitTunnel, PubkeyAuthentication,
411 RhostsRSAAuthentication, RSAAuthentication, X11DisplayOffset,
412 X11Forwarding and X11UseLocalHost.
415 Specifies the maximum number of authentication attempts permitted
416 per connection. Once the number of failures reaches half this
417 value, additional failures are logged. The default is 6.
420 Specifies the maximum number of open sessions permitted per
421 network connection. The default is 10.
424 Specifies the maximum number of concurrent unauthenticated
425 connections to the SSH daemon. Additional connections will be
426 dropped until authentication succeeds or the LoginGraceTime
427 expires for a connection. The default is 10.
429 Alternatively, random early drop can be enabled by specifying the
430 three colon separated values ``start:rate:full'' (e.g.
431 "10:30:60"). sshd(8) will refuse connection attempts with a
432 probability of ``rate/100'' (30%) if there are currently
433 ``start'' (10) unauthenticated connections. The probability
434 increases linearly and all connection attempts are refused if the
435 number of unauthenticated connections reaches ``full'' (60).
437 PasswordAuthentication
438 Specifies whether password authentication is allowed. The
442 When password authentication is allowed, it specifies whether the
443 server allows login to accounts with empty password strings. The
447 Specifies the destinations to which TCP port forwarding is
448 permitted. The forwarding specification must be one of the
452 PermitOpen IPv4_addr:port
453 PermitOpen [IPv6_addr]:port
455 Multiple forwards may be specified by separating them with
456 whitespace. An argument of ``any'' can be used to remove all
457 restrictions and permit any forwarding requests. By default all
458 port forwarding requests are permitted.
461 Specifies whether root can log in using ssh(1). The argument
462 must be ``yes'', ``without-password'', ``forced-commands-only'',
463 or ``no''. The default is ``yes''.
465 If this option is set to ``without-password'', password
466 authentication is disabled for root.
468 If this option is set to ``forced-commands-only'', root login
469 with public key authentication will be allowed, but only if the
470 command option has been specified (which may be useful for taking
471 remote backups even if root login is normally not allowed). All
472 other authentication methods are disabled for root.
474 If this option is set to ``no'', root is not allowed to log in.
477 Specifies whether tun(4) device forwarding is allowed. The
478 argument must be ``yes'', ``point-to-point'' (layer 3),
479 ``ethernet'' (layer 2), or ``no''. Specifying ``yes'' permits
480 both ``point-to-point'' and ``ethernet''. The default is ``no''.
482 PermitUserEnvironment
483 Specifies whether ~/.ssh/environment and environment= options in
484 ~/.ssh/authorized_keys are processed by sshd(8). The default is
485 ``no''. Enabling environment processing may enable users to
486 bypass access restrictions in some configurations using
487 mechanisms such as LD_PRELOAD.
490 Specifies the file that contains the process ID of the SSH
491 daemon. The default is /var/run/sshd.pid.
493 Port Specifies the port number that sshd(8) listens on. The default
494 is 22. Multiple options of this type are permitted. See also
498 Specifies whether sshd(8) should print the date and time of the
499 last user login when a user logs in interactively. The default
503 Specifies whether sshd(8) should print /etc/motd when a user logs
504 in interactively. (On some systems it is also printed by the
505 shell, /etc/profile, or equivalent.) The default is ``yes''.
508 Specifies the protocol versions sshd(8) supports. The possible
509 values are `1' and `2'. Multiple versions must be comma-
510 separated. The default is `2'. Note that the order of the
511 protocol list does not indicate preference, because the client
512 selects among multiple protocol versions offered by the server.
513 Specifying ``2,1'' is identical to ``1,2''.
516 Specifies whether public key authentication is allowed. The
517 default is ``yes''. Note that this option applies to protocol
521 Specifies a list of revoked public keys. Keys listed in this
522 file will be refused for public key authentication. Note that if
523 this file is not readable, then public key authentication will be
524 refused for all users.
526 RhostsRSAAuthentication
527 Specifies whether rhosts or /etc/hosts.equiv authentication
528 together with successful RSA host authentication is allowed. The
529 default is ``no''. This option applies to protocol version 1
533 Specifies whether pure RSA authentication is allowed. The
534 default is ``yes''. This option applies to protocol version 1
538 Defines the number of bits in the ephemeral protocol version 1
539 server key. The minimum value is 512, and the default is 1024.
542 Specifies whether sshd(8) should check file modes and ownership
543 of the user's files and home directory before accepting login.
544 This is normally desirable because novices sometimes accidentally
545 leave their directory or files world-writable. The default is
546 ``yes''. Note that this does not apply to ChrootDirectory, whose
547 permissions and ownership are checked unconditionally.
550 Configures an external subsystem (e.g. file transfer daemon).
551 Arguments should be a subsystem name and a command (with optional
552 arguments) to execute upon subsystem request.
554 The command sftp-server(8) implements the ``sftp'' file transfer
557 Alternately the name ``internal-sftp'' implements an in-process
558 ``sftp'' server. This may simplify configurations using
559 ChrootDirectory to force a different filesystem root on clients.
561 By default no subsystems are defined. Note that this option
562 applies to protocol version 2 only.
565 Gives the facility code that is used when logging messages from
566 sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0,
567 LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The
571 Specifies whether the system should send TCP keepalive messages
572 to the other side. If they are sent, death of the connection or
573 crash of one of the machines will be properly noticed. However,
574 this means that connections will die if the route is down
575 temporarily, and some people find it annoying. On the other
576 hand, if TCP keepalives are not sent, sessions may hang
577 indefinitely on the server, leaving ``ghost'' users and consuming
580 The default is ``yes'' (to send TCP keepalive messages), and the
581 server will notice if the network goes down or the client host
582 crashes. This avoids infinitely hanging sessions.
584 To disable TCP keepalive messages, the value should be set to
588 Specifies a file containing public keys of certificate
589 authorities that are trusted to sign user certificates for
590 authentication. Keys are listed one per line; empty lines and
591 comments starting with `#' are allowed. If a certificate is
592 presented for authentication and has its signing CA key listed in
593 this file, then it may be used for authentication for any user
594 listed in the certificate's principals list. Note that
595 certificates that lack a list of principals will not be permitted
596 for authentication using TrustedUserCAKeys. For more details on
597 certificates, see the CERTIFICATES section in ssh-keygen(1).
599 UseDNS Specifies whether sshd(8) should look up the remote host name and
600 check that the resolved host name for the remote IP address maps
601 back to the very same IP address. The default is ``yes''.
604 Specifies whether login(1) is used for interactive login
605 sessions. The default is ``no''. Note that login(1) is never
606 used for remote command execution. Note also, that if this is
607 enabled, X11Forwarding will be disabled because login(1) does not
608 know how to handle xauth(1) cookies. If UsePrivilegeSeparation
609 is specified, it will be disabled after authentication.
611 UsePAM Enables the Pluggable Authentication Module interface. If set to
612 ``yes'' this will enable PAM authentication using
613 ChallengeResponseAuthentication and PasswordAuthentication in
614 addition to PAM account and session module processing for all
615 authentication types.
617 Because PAM challenge-response authentication usually serves an
618 equivalent role to password authentication, you should disable
619 either PasswordAuthentication or ChallengeResponseAuthentication.
621 If UsePAM is enabled, you will not be able to run sshd(8) as a
622 non-root user. The default is ``no''.
624 UsePrivilegeSeparation
625 Specifies whether sshd(8) separates privileges by creating an
626 unprivileged child process to deal with incoming network traffic.
627 After successful authentication, another process will be created
628 that has the privilege of the authenticated user. The goal of
629 privilege separation is to prevent privilege escalation by
630 containing any corruption within the unprivileged processes. The
631 default is ``yes''. If UsePrivilegeSeparation is set to
632 ``sandbox'' then the pre-authentication unprivileged process is
633 subject to additional restrictions.
636 Specifies the first display number available for sshd(8)'s X11
637 forwarding. This prevents sshd from interfering with real X11
638 servers. The default is 10.
641 Specifies whether X11 forwarding is permitted. The argument must
642 be ``yes'' or ``no''. The default is ``no''.
644 When X11 forwarding is enabled, there may be additional exposure
645 to the server and to client displays if the sshd(8) proxy display
646 is configured to listen on the wildcard address (see
647 X11UseLocalhost below), though this is not the default.
648 Additionally, the authentication spoofing and authentication data
649 verification and substitution occur on the client side. The
650 security risk of using X11 forwarding is that the client's X11
651 display server may be exposed to attack when the SSH client
652 requests forwarding (see the warnings for ForwardX11 in
653 ssh_config(5)). A system administrator may have a stance in
654 which they want to protect clients that may expose themselves to
655 attack by unwittingly requesting X11 forwarding, which can
656 warrant a ``no'' setting.
658 Note that disabling X11 forwarding does not prevent users from
659 forwarding X11 traffic, as users can always install their own
660 forwarders. X11 forwarding is automatically disabled if UseLogin
664 Specifies whether sshd(8) should bind the X11 forwarding server
665 to the loopback address or to the wildcard address. By default,
666 sshd binds the forwarding server to the loopback address and sets
667 the hostname part of the DISPLAY environment variable to
668 ``localhost''. This prevents remote hosts from connecting to the
669 proxy display. However, some older X11 clients may not function
670 with this configuration. X11UseLocalhost may be set to ``no'' to
671 specify that the forwarding server should be bound to the
672 wildcard address. The argument must be ``yes'' or ``no''. The
676 Specifies the full pathname of the xauth(1) program. The default
677 is /usr/X11R6/bin/xauth.
680 sshd(8) command-line arguments and configuration file options that
681 specify time may be expressed using a sequence of the form:
682 time[qualifier], where time is a positive integer value and qualifier is
683 one of the following:
692 Each member of the sequence is added together to calculate the total time
695 Time format examples:
697 600 600 seconds (10 minutes)
699 1h30m 1 hour 30 minutes (90 minutes)
703 Contains configuration data for sshd(8). This file should be
704 writable by root only, but it is recommended (though not
705 necessary) that it be world-readable.
711 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
712 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
713 de Raadt and Dug Song removed many bugs, re-added newer features and
714 created OpenSSH. Markus Friedl contributed the support for SSH protocol
715 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support
716 for privilege separation.
718 OpenBSD 5.0 August 2, 2011 OpenBSD 5.0
projects / openssh.git / blob