sshd_config.0 revision 204861
1SSHD_CONFIG(5) OpenBSD Programmer's Manual SSHD_CONFIG(5) 2 3NAME 4 sshd_config - 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-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 16 The possible keywords and their meanings are as follows (note that key- 17 words 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 envi- 23 ronment passing is only supported for protocol 2. Variables are 24 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. 31 32 AddressFamily 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''. 36 37 AllowAgentForwarding 38 Specifies whether ssh-agent(1) forwarding is permitted. The de- 39 fault is ``yes''. Note that disabling agent forwarding does not 40 improve security unless users are also denied shell access, as 41 they can always install their own forwarders. 42 43 AllowGroups 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 51 AllowGroups. 52 53 See PATTERNS in ssh_config(5) for more information on patterns. 54 55 AllowTcpForwarding 56 Specifies whether TCP forwarding is permitted. The default is 57 ``yes''. Note that disabling TCP forwarding does not improve se- 58 curity unless users are also denied shell access, as they can al- 59 ways install their own forwarders. 60 61 AllowUsers 62 This keyword can be followed by a list of user name patterns, 63 separated by spaces. If specified, login is allowed only for us- 64 er 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 US- 67 ER@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. 71 72 See PATTERNS in ssh_config(5) for more information on patterns. 73 74 AuthorizedKeysFile 75 Specifies the file that contains the public keys that can be used 76 for user authentication. AuthorizedKeysFile may contain tokens 77 of the form %T which are substituted during connection setup. 78 The following tokens are defined: %% is replaced by a literal 79 '%', %h is replaced by the home directory of the user being au- 80 thenticated, and %u is replaced by the username of that user. 81 After expansion, AuthorizedKeysFile is taken to be an absolute 82 path or one relative to the user's home directory. The default 83 is ``.ssh/authorized_keys''. 84 85 Banner The contents of the specified file are sent to the remote user 86 before authentication is allowed. If the argument is ``none'' 87 then no banner is displayed. This option is only available for 88 protocol version 2. By default, no banner is displayed. 89 90 ChallengeResponseAuthentication 91 Specifies whether challenge-response authentication is allowed 92 (e.g. via PAM or though authentication styles supported in 93 login.conf(5)) The default is ``yes''. 94 95 ChrootDirectory 96 Specifies the pathname of a directory to chroot(2) to after au- 97 thentication. All components of the pathname must be root-owned 98 directories that are not writable by any other user or group. 99 After the chroot, sshd(8) changes the working directory to the 100 user's home directory. 101 102 The pathname may contain the following tokens that are expanded 103 at runtime once the connecting user has been authenticated: %% is 104 replaced by a literal '%', %h is replaced by the home directory 105 of the user being authenticated, and %u is replaced by the user- 106 name of that user. 107 108 The ChrootDirectory must contain the necessary files and directo- 109 ries to support the user's session. For an interactive session 110 this requires at least a shell, typically sh(1), and basic /dev 111 nodes such as null(4), zero(4), stdin(4), stdout(4), stderr(4), 112 arandom(4) and tty(4) devices. For file transfer sessions using 113 ``sftp'', no additional configuration of the environment is nec- 114 essary if the in-process sftp server is used, though sessions 115 which use logging do require /dev/log inside the chroot directory 116 (see sftp-server(8) for details). 117 118 The default is not to chroot(2). 119 120 Ciphers 121 Specifies the ciphers allowed for protocol version 2. Multiple 122 ciphers must be comma-separated. The supported ciphers are 123 ``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'', ``aes256-cbc'', 124 ``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'', ``arcfour128'', 125 ``arcfour256'', ``arcfour'', ``blowfish-cbc'', and 126 ``cast128-cbc''. The default is: 127 128 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 129 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 130 aes256-cbc,arcfour 131 132 ClientAliveCountMax 133 Sets the number of client alive messages (see below) which may be 134 sent without sshd(8) receiving any messages back from the client. 135 If this threshold is reached while client alive messages are be- 136 ing sent, sshd will disconnect the client, terminating the ses- 137 sion. It is important to note that the use of client alive mes- 138 sages is very different from TCPKeepAlive (below). The client 139 alive messages are sent through the encrypted channel and there- 140 fore will not be spoofable. The TCP keepalive option enabled by 141 TCPKeepAlive is spoofable. The client alive mechanism is valu- 142 able when the client or server depend on knowing when a connec- 143 tion has become inactive. 144 145 The default value is 3. If ClientAliveInterval (see below) is 146 set to 15, and ClientAliveCountMax is left at the default, unre- 147 sponsive SSH clients will be disconnected after approximately 45 148 seconds. This option applies to protocol version 2 only. 149 150 ClientAliveInterval 151 Sets a timeout interval in seconds after which if no data has 152 been received from the client, sshd(8) will send a message 153 through the encrypted channel to request a response from the 154 client. The default is 0, indicating that these messages will 155 not be sent to the client. This option applies to protocol ver- 156 sion 2 only. 157 158 Compression 159 Specifies whether compression is allowed, or delayed until the 160 user has authenticated successfully. The argument must be 161 ``yes'', ``delayed'', or ``no''. The default is ``delayed''. 162 163 DenyGroups 164 This keyword can be followed by a list of group name patterns, 165 separated by spaces. Login is disallowed for users whose primary 166 group or supplementary group list matches one of the patterns. 167 Only group names are valid; a numerical group ID is not recog- 168 nized. By default, login is allowed for all groups. The al- 169 low/deny directives are processed in the following order: 170 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups. 171 172 See PATTERNS in ssh_config(5) for more information on patterns. 173 174 DenyUsers 175 This keyword can be followed by a list of user name patterns, 176 separated by spaces. Login is disallowed for user names that 177 match one of the patterns. Only user names are valid; a numeri- 178 cal user ID is not recognized. By default, login is allowed for 179 all users. If the pattern takes the form USER@HOST then USER and 180 HOST are separately checked, restricting logins to particular 181 users from particular hosts. The allow/deny directives are pro- 182 cessed in the following order: DenyUsers, AllowUsers, DenyGroups, 183 and finally AllowGroups. 184 185 See PATTERNS in ssh_config(5) for more information on patterns. 186 187 ForceCommand 188 Forces the execution of the command specified by ForceCommand, 189 ignoring any command supplied by the client and ~/.ssh/rc if pre- 190 sent. The command is invoked by using the user's login shell 191 with the -c option. This applies to shell, command, or subsystem 192 execution. It is most useful inside a Match block. The command 193 originally supplied by the client is available in the 194 SSH_ORIGINAL_COMMAND environment variable. Specifying a command 195 of ``internal-sftp'' will force the use of an in-process sftp 196 server that requires no support files when used with 197 ChrootDirectory. 198 199 GatewayPorts 200 Specifies whether remote hosts are allowed to connect to ports 201 forwarded for the client. By default, sshd(8) binds remote port 202 forwardings to the loopback address. This prevents other remote 203 hosts from connecting to forwarded ports. GatewayPorts can be 204 used to specify that sshd should allow remote port forwardings to 205 bind to non-loopback addresses, thus allowing other hosts to con- 206 nect. The argument may be ``no'' to force remote port forward- 207 ings to be available to the local host only, ``yes'' to force re- 208 mote port forwardings to bind to the wildcard address, or 209 ``clientspecified'' to allow the client to select the address to 210 which the forwarding is bound. The default is ``no''. 211 212 GSSAPIAuthentication 213 Specifies whether user authentication based on GSSAPI is allowed. 214 The default is ``no''. Note that this option applies to protocol 215 version 2 only. 216 217 GSSAPICleanupCredentials 218 Specifies whether to automatically destroy the user's credentials 219 cache on logout. The default is ``yes''. Note that this option 220 applies to protocol version 2 only. 221 222 HostbasedAuthentication 223 Specifies whether rhosts or /etc/hosts.equiv authentication to- 224 gether with successful public key client host authentication is 225 allowed (host-based authentication). This option is similar to 226 RhostsRSAAuthentication and applies to protocol version 2 only. 227 The default is ``no''. 228 229 HostbasedUsesNameFromPacketOnly 230 Specifies whether or not the server will attempt to perform a re- 231 verse name lookup when matching the name in the ~/.shosts, 232 ~/.rhosts, and /etc/hosts.equiv files during 233 HostbasedAuthentication. A setting of ``yes'' means that sshd(8) 234 uses the name supplied by the client rather than attempting to 235 resolve the name from the TCP connection itself. The default is 236 ``no''. 237 238 HostCertificate 239 Specifies a file containing a public host certificate. The cer- 240 tificate's public key must match a private host key already spec- 241 ified by HostKey. The default behaviour of sshd(8) is not to 242 load any certificates. 243 244 HostKey 245 Specifies a file containing a private host key used by SSH. The 246 default is /etc/ssh/ssh_host_key for protocol version 1, and 247 /etc/ssh/ssh_host_rsa_key and /etc/ssh/ssh_host_dsa_key for pro- 248 tocol version 2. Note that sshd(8) will refuse to use a file if 249 it is group/world-accessible. It is possible to have multiple 250 host key files. ``rsa1'' keys are used for version 1 and ``dsa'' 251 or ``rsa'' are used for version 2 of the SSH protocol. 252 253 IgnoreRhosts 254 Specifies that .rhosts and .shosts files will not be used in 255 RhostsRSAAuthentication or HostbasedAuthentication. 256 257 /etc/hosts.equiv and /etc/shosts.equiv are still used. The de- 258 fault is ``yes''. 259 260 IgnoreUserKnownHosts 261 Specifies whether sshd(8) should ignore the user's 262 ~/.ssh/known_hosts during RhostsRSAAuthentication or 263 HostbasedAuthentication. The default is ``no''. 264 265 KerberosAuthentication 266 Specifies whether the password provided by the user for 267 PasswordAuthentication will be validated through the Kerberos 268 KDC. To use this option, the server needs a Kerberos servtab 269 which allows the verification of the KDC's identity. The default 270 is ``no''. 271 272 KerberosGetAFSToken 273 If AFS is active and the user has a Kerberos 5 TGT, attempt to 274 acquire an AFS token before accessing the user's home directory. 275 The default is ``no''. 276 277 KerberosOrLocalPasswd 278 If password authentication through Kerberos fails then the pass- 279 word will be validated via any additional local mechanism such as 280 /etc/passwd. The default is ``yes''. 281 282 KerberosTicketCleanup 283 Specifies whether to automatically destroy the user's ticket 284 cache file on logout. The default is ``yes''. 285 286 KeyRegenerationInterval 287 In protocol version 1, the ephemeral server key is automatically 288 regenerated after this many seconds (if it has been used). The 289 purpose of regeneration is to prevent decrypting captured ses- 290 sions by later breaking into the machine and stealing the keys. 291 The key is never stored anywhere. If the value is 0, the key is 292 never regenerated. The default is 3600 (seconds). 293 294 ListenAddress 295 Specifies the local addresses sshd(8) should listen on. The fol- 296 lowing forms may be used: 297 298 ListenAddress host|IPv4_addr|IPv6_addr 299 ListenAddress host|IPv4_addr:port 300 ListenAddress [host|IPv6_addr]:port 301 302 If port is not specified, sshd will listen on the address and all 303 prior Port options specified. The default is to listen on all 304 local addresses. Multiple ListenAddress options are permitted. 305 Additionally, any Port options must precede this option for non- 306 port qualified addresses. 307 308 LoginGraceTime 309 The server disconnects after this time if the user has not suc- 310 cessfully logged in. If the value is 0, there is no time limit. 311 The default is 120 seconds. 312 313 LogLevel 314 Gives the verbosity level that is used when logging messages from 315 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO, 316 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO. 317 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify 318 higher levels of debugging output. Logging with a DEBUG level 319 violates the privacy of users and is not recommended. 320 321 MACs Specifies the available MAC (message authentication code) algo- 322 rithms. The MAC algorithm is used in protocol version 2 for data 323 integrity protection. Multiple algorithms must be comma-separat- 324 ed. The default is: 325 326 hmac-md5,hmac-sha1,umac-64@openssh.com, 327 hmac-ripemd160,hmac-sha1-96,hmac-md5-96 328 329 Match Introduces a conditional block. If all of the criteria on the 330 Match line are satisfied, the keywords on the following lines 331 override those set in the global section of the config file, un- 332 til either another Match line or the end of the file. 333 334 The arguments to Match are one or more criteria-pattern pairs. 335 The available criteria are User, Group, Host, and Address. The 336 match patterns may consist of single entries or comma-separated 337 lists and may use the wildcard and negation operators described 338 in the PATTERNS section of ssh_config(5). 339 340 The patterns in an Address criteria may additionally contain ad- 341 dresses to match in CIDR address/masklen format, e.g. 342 ``192.0.2.0/24'' or ``3ffe:ffff::/32''. Note that the mask 343 length provided must be consistent with the address - it is an 344 error to specify a mask length that is too long for the address 345 or one with bits set in this host portion of the address. For 346 example, ``192.0.2.0/33'' and ``192.0.2.0/8'' respectively. 347 348 Only a subset of keywords may be used on the lines following a 349 Match keyword. Available keywords are AllowAgentForwarding, 350 AllowTcpForwarding, Banner, ChrootDirectory, ForceCommand, 351 GatewayPorts, GSSAPIAuthentication, HostbasedAuthentication, 352 KbdInteractiveAuthentication, KerberosAuthentication, 353 MaxAuthTries, MaxSessions, PasswordAuthentication, 354 PermitEmptyPasswords, PermitOpen, PermitRootLogin, 355 PubkeyAuthentication, RhostsRSAAuthentication, RSAAuthentication, 356 X11DisplayOffset, X11Forwarding and X11UseLocalHost. 357 358 MaxAuthTries 359 Specifies the maximum number of authentication attempts permitted 360 per connection. Once the number of failures reaches half this 361 value, additional failures are logged. The default is 6. 362 363 MaxSessions 364 Specifies the maximum number of open sessions permitted per net- 365 work connection. The default is 10. 366 367 MaxStartups 368 Specifies the maximum number of concurrent unauthenticated con- 369 nections to the SSH daemon. Additional connections will be 370 dropped until authentication succeeds or the LoginGraceTime ex- 371 pires for a connection. The default is 10. 372 373 Alternatively, random early drop can be enabled by specifying the 374 three colon separated values ``start:rate:full'' (e.g. 375 "10:30:60"). sshd(8) will refuse connection attempts with a 376 probability of ``rate/100'' (30%) if there are currently 377 ``start'' (10) unauthenticated connections. The probability in- 378 creases linearly and all connection attempts are refused if the 379 number of unauthenticated connections reaches ``full'' (60). 380 381 PasswordAuthentication 382 Specifies whether password authentication is allowed. The de- 383 fault is ``yes''. 384 385 PermitEmptyPasswords 386 When password authentication is allowed, it specifies whether the 387 server allows login to accounts with empty password strings. The 388 default is ``no''. 389 390 PermitOpen 391 Specifies the destinations to which TCP port forwarding is per- 392 mitted. The forwarding specification must be one of the follow- 393 ing forms: 394 395 PermitOpen host:port 396 PermitOpen IPv4_addr:port 397 PermitOpen [IPv6_addr]:port 398 399 Multiple forwards may be specified by separating them with 400 whitespace. An argument of ``any'' can be used to remove all re- 401 strictions and permit any forwarding requests. By default all 402 port forwarding requests are permitted. 403 404 PermitRootLogin 405 Specifies whether root can log in using ssh(1). The argument 406 must be ``yes'', ``without-password'', ``forced-commands-only'', 407 or ``no''. The default is ``yes''. 408 409 If this option is set to ``without-password'', password authenti- 410 cation is disabled for root. 411 412 If this option is set to ``forced-commands-only'', root login 413 with public key authentication will be allowed, but only if the 414 command option has been specified (which may be useful for taking 415 remote backups even if root login is normally not allowed). All 416 other authentication methods are disabled for root. 417 418 If this option is set to ``no'', root is not allowed to log in. 419 420 PermitTunnel 421 Specifies whether tun(4) device forwarding is allowed. The argu- 422 ment must be ``yes'', ``point-to-point'' (layer 3), ``ethernet'' 423 (layer 2), or ``no''. Specifying ``yes'' permits both ``point- 424 to-point'' and ``ethernet''. The default is ``no''. 425 426 PermitUserEnvironment 427 Specifies whether ~/.ssh/environment and environment= options in 428 ~/.ssh/authorized_keys are processed by sshd(8). The default is 429 ``no''. Enabling environment processing may enable users to by- 430 pass access restrictions in some configurations using mechanisms 431 such as LD_PRELOAD. 432 433 PidFile 434 Specifies the file that contains the process ID of the SSH dae- 435 mon. The default is /var/run/sshd.pid. 436 437 Port Specifies the port number that sshd(8) listens on. The default 438 is 22. Multiple options of this type are permitted. See also 439 ListenAddress. 440 441 PrintLastLog 442 Specifies whether sshd(8) should print the date and time of the 443 last user login when a user logs in interactively. The default 444 is ``yes''. 445 446 PrintMotd 447 Specifies whether sshd(8) should print /etc/motd when a user logs 448 in interactively. (On some systems it is also printed by the 449 shell, /etc/profile, or equivalent.) The default is ``yes''. 450 451 Protocol 452 Specifies the protocol versions sshd(8) supports. The possible 453 values are `1' and `2'. Multiple versions must be comma-separat- 454 ed. The default is `2'. Note that the order of the protocol 455 list does not indicate preference, because the client selects 456 among multiple protocol versions offered by the server. Specify- 457 ing ``2,1'' is identical to ``1,2''. 458 459 PubkeyAuthentication 460 Specifies whether public key authentication is allowed. The de- 461 fault is ``yes''. Note that this option applies to protocol ver- 462 sion 2 only. 463 464 RevokedKeys 465 Specifies a list of revoked public keys. Keys listed in this 466 file will be refused for public key authentication. Note that if 467 this file is not readable, then public key authentication will be 468 refused for all users. 469 470 RhostsRSAAuthentication 471 Specifies whether rhosts or /etc/hosts.equiv authentication to- 472 gether with successful RSA host authentication is allowed. The 473 default is ``no''. This option applies to protocol version 1 on- 474 ly. 475 476 RSAAuthentication 477 Specifies whether pure RSA authentication is allowed. The de- 478 fault is ``yes''. This option applies to protocol version 1 on- 479 ly. 480 481 ServerKeyBits 482 Defines the number of bits in the ephemeral protocol version 1 483 server key. The minimum value is 512, and the default is 1024. 484 485 StrictModes 486 Specifies whether sshd(8) should check file modes and ownership 487 of the user's files and home directory before accepting login. 488 This is normally desirable because novices sometimes accidentally 489 leave their directory or files world-writable. The default is 490 ``yes''. Note that this does not apply to ChrootDirectory, whose 491 permissions and ownership are checked unconditionally. 492 493 Subsystem 494 Configures an external subsystem (e.g. file transfer daemon). 495 Arguments should be a subsystem name and a command (with optional 496 arguments) to execute upon subsystem request. 497 498 The command sftp-server(8) implements the ``sftp'' file transfer 499 subsystem. 500 501 Alternately the name ``internal-sftp'' implements an in-process 502 ``sftp'' server. This may simplify configurations using 503 ChrootDirectory to force a different filesystem root on clients. 504 505 By default no subsystems are defined. Note that this option ap- 506 plies to protocol version 2 only. 507 508 SyslogFacility 509 Gives the facility code that is used when logging messages from 510 sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0, 511 LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The de- 512 fault is AUTH. 513 514 TCPKeepAlive 515 Specifies whether the system should send TCP keepalive messages 516 to the other side. If they are sent, death of the connection or 517 crash of one of the machines will be properly noticed. However, 518 this means that connections will die if the route is down tem- 519 porarily, and some people find it annoying. On the other hand, 520 if TCP keepalives are not sent, sessions may hang indefinitely on 521 the server, leaving ``ghost'' users and consuming server re- 522 sources. 523 524 The default is ``yes'' (to send TCP keepalive messages), and the 525 server will notice if the network goes down or the client host 526 crashes. This avoids infinitely hanging sessions. 527 528 To disable TCP keepalive messages, the value should be set to 529 ``no''. 530 531 TrustedUserCAKeys 532 Specifies a file containing public keys of certificate authori- 533 ties that are trusted to sign user certificates for authentica- 534 tion. Keys are listed one per line; empty lines and comments 535 starting with `#' are allowed. If a certificate is presented for 536 authentication and has its signing CA key listed in this file, 537 then it may be used for authentication for any user listed in the 538 certificate's principals list. Note that certificates that lack 539 a list of principals will not be permitted for authentication us- 540 ing TrustedUserCAKeys. For more details on certificates, see the 541 CERTIFICATES section in ssh-keygen(1). 542 543 UseDNS Specifies whether sshd(8) should look up the remote host name and 544 check that the resolved host name for the remote IP address maps 545 back to the very same IP address. The default is ``yes''. 546 547 UseLogin 548 Specifies whether login(1) is used for interactive login ses- 549 sions. The default is ``no''. Note that login(1) is never used 550 for remote command execution. Note also, that if this is en- 551 abled, X11Forwarding will be disabled because login(1) does not 552 know how to handle xauth(1) cookies. If UsePrivilegeSeparation 553 is specified, it will be disabled after authentication. 554 555 UsePAM Enables the Pluggable Authentication Module interface. If set to 556 ``yes'' this will enable PAM authentication using 557 ChallengeResponseAuthentication and PasswordAuthentication in ad- 558 dition to PAM account and session module processing for all au- 559 thentication types. 560 561 Because PAM challenge-response authentication usually serves an 562 equivalent role to password authentication, you should disable 563 either PasswordAuthentication or ChallengeResponseAuthentication. 564 565 If UsePAM is enabled, you will not be able to run sshd(8) as a 566 non-root user. The default is ``no''. 567 568 UsePrivilegeSeparation 569 Specifies whether sshd(8) separates privileges by creating an un- 570 privileged child process to deal with incoming network traffic. 571 After successful authentication, another process will be created 572 that has the privilege of the authenticated user. The goal of 573 privilege separation is to prevent privilege escalation by con- 574 taining any corruption within the unprivileged processes. The 575 default is ``yes''. 576 577 X11DisplayOffset 578 Specifies the first display number available for sshd(8)'s X11 579 forwarding. This prevents sshd from interfering with real X11 580 servers. The default is 10. 581 582 X11Forwarding 583 Specifies whether X11 forwarding is permitted. The argument must 584 be ``yes'' or ``no''. The default is ``no''. 585 586 When X11 forwarding is enabled, there may be additional exposure 587 to the server and to client displays if the sshd(8) proxy display 588 is configured to listen on the wildcard address (see 589 X11UseLocalhost below), though this is not the default. Addi- 590 tionally, the authentication spoofing and authentication data 591 verification and substitution occur on the client side. The se- 592 curity risk of using X11 forwarding is that the client's X11 dis- 593 play server may be exposed to attack when the SSH client requests 594 forwarding (see the warnings for ForwardX11 in ssh_config(5)). A 595 system administrator may have a stance in which they want to pro- 596 tect clients that may expose themselves to attack by unwittingly 597 requesting X11 forwarding, which can warrant a ``no'' setting. 598 599 Note that disabling X11 forwarding does not prevent users from 600 forwarding X11 traffic, as users can always install their own 601 forwarders. X11 forwarding is automatically disabled if UseLogin 602 is enabled. 603 604 X11UseLocalhost 605 Specifies whether sshd(8) should bind the X11 forwarding server 606 to the loopback address or to the wildcard address. By default, 607 sshd binds the forwarding server to the loopback address and sets 608 the hostname part of the DISPLAY environment variable to 609 ``localhost''. This prevents remote hosts from connecting to the 610 proxy display. However, some older X11 clients may not function 611 with this configuration. X11UseLocalhost may be set to ``no'' to 612 specify that the forwarding server should be bound to the wild- 613 card address. The argument must be ``yes'' or ``no''. The de- 614 fault is ``yes''. 615 616 XAuthLocation 617 Specifies the full pathname of the xauth(1) program. The default 618 is /usr/X11R6/bin/xauth. 619 620TIME FORMATS 621 sshd(8) command-line arguments and configuration file options that speci- 622 fy time may be expressed using a sequence of the form: time[qualifier], 623 where time is a positive integer value and qualifier is one of the fol- 624 lowing: 625 626 <none> seconds 627 s | S seconds 628 m | M minutes 629 h | H hours 630 d | D days 631 w | W weeks 632 633 Each member of the sequence is added together to calculate the total time 634 value. 635 636 Time format examples: 637 638 600 600 seconds (10 minutes) 639 10m 10 minutes 640 1h30m 1 hour 30 minutes (90 minutes) 641 642FILES 643 /etc/ssh/sshd_config 644 Contains configuration data for sshd(8). This file should be 645 writable by root only, but it is recommended (though not neces- 646 sary) that it be world-readable. 647 648SEE ALSO 649 sshd(8) 650 651AUTHORS 652 OpenSSH is a derivative of the original and free ssh 1.2.12 release by 653 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 654 de Raadt and Dug Song removed many bugs, re-added newer features and cre- 655 ated OpenSSH. Markus Friedl contributed the support for SSH protocol 656 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support 657 for privilege separation. 658 659OpenBSD 4.6 March 4, 2010 10 660