sshd_config.0 revision 218767
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- 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. 15 16 The possible keywords and their meanings are as follows (note that 17 keywords are case-insensitive and arguments are case-sensitive): 18 19 AcceptEnv 20 Specifies what environment variables sent by the client will be 21 copied into the session's environ(7). See SendEnv in 22 ssh_config(5) for how to configure the client. Note that 23 environment passing is only supported for protocol 2. 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. 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 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. 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 58 security unless users are also denied shell access, as they can 59 always 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 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. 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. 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. The default is 85 ``.ssh/authorized_keys''. 86 87 AuthorizedPrincipalsFile 88 Specifies a file that lists principal names that are accepted for 89 certificate authentication. When using certificates signed by a 90 key listed in TrustedUserCAKeys, this file lists names, one of 91 which must appear in the certificate for it to be accepted for 92 authentication. Names are listed one per line preceded by key 93 options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)). 94 Empty lines and comments starting with `#' are ignored. 95 96 AuthorizedPrincipalsFile may contain tokens of the form %T which 97 are substituted during connection setup. The following tokens 98 are defined: %% is replaced by a literal '%', %h is replaced by 99 the home directory of the user being authenticated, and %u is 100 replaced by the username of that user. After expansion, 101 AuthorizedPrincipalsFile is taken to be an absolute path or one 102 relative to the user's home directory. 103 104 The default is not to use a principals file - in this case, the 105 username of the user must appear in a certificate's principals 106 list for it to be accepted. Note that AuthorizedPrincipalsFile 107 is only used when authentication proceeds using a CA listed in 108 TrustedUserCAKeys and is not consulted for certification 109 authorities trusted via ~/.ssh/authorized_keys, though the 110 principals= key option offers a similar facility (see sshd(8) for 111 details). 112 113 Banner The contents of the specified file are sent to the remote user 114 before authentication is allowed. If the argument is ``none'' 115 then no banner is displayed. This option is only available for 116 protocol version 2. By default, no banner is displayed. 117 118 ChallengeResponseAuthentication 119 Specifies whether challenge-response authentication is allowed 120 (e.g. via PAM or though authentication styles supported in 121 login.conf(5)) The default is ``yes''. 122 123 ChrootDirectory 124 Specifies the pathname of a directory to chroot(2) to after 125 authentication. All components of the pathname must be root- 126 owned directories that are not writable by any other user or 127 group. After the chroot, sshd(8) changes the working directory 128 to the user's home directory. 129 130 The pathname may contain the following tokens that are expanded 131 at runtime once the connecting user has been authenticated: %% is 132 replaced by a literal '%', %h is replaced by the home directory 133 of the user being authenticated, and %u is replaced by the 134 username of that user. 135 136 The ChrootDirectory must contain the necessary files and 137 directories to support the user's session. For an interactive 138 session this requires at least a shell, typically sh(1), and 139 basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4), 140 stderr(4), arandom(4) and tty(4) devices. For file transfer 141 sessions using ``sftp'', no additional configuration of the 142 environment is necessary if the in-process sftp server is used, 143 though sessions which use logging do require /dev/log inside the 144 chroot directory (see sftp-server(8) for details). 145 146 The default is not to chroot(2). 147 148 Ciphers 149 Specifies the ciphers allowed for protocol version 2. Multiple 150 ciphers must be comma-separated. The supported ciphers are 151 ``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'', ``aes256-cbc'', 152 ``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'', ``arcfour128'', 153 ``arcfour256'', ``arcfour'', ``blowfish-cbc'', and 154 ``cast128-cbc''. The default is: 155 156 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 157 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 158 aes256-cbc,arcfour 159 160 ClientAliveCountMax 161 Sets the number of client alive messages (see below) which may be 162 sent without sshd(8) receiving any messages back from the client. 163 If this threshold is reached while client alive messages are 164 being sent, sshd will disconnect the client, terminating the 165 session. It is important to note that the use of client alive 166 messages is very different from TCPKeepAlive (below). The client 167 alive messages are sent through the encrypted channel and 168 therefore will not be spoofable. The TCP keepalive option 169 enabled by TCPKeepAlive is spoofable. The client alive mechanism 170 is valuable when the client or server depend on knowing when a 171 connection has become inactive. 172 173 The default value is 3. If ClientAliveInterval (see below) is 174 set to 15, and ClientAliveCountMax is left at the default, 175 unresponsive SSH clients will be disconnected after approximately 176 45 seconds. This option applies to protocol version 2 only. 177 178 ClientAliveInterval 179 Sets a timeout interval in seconds after which if no data has 180 been received from the client, sshd(8) will send a message 181 through the encrypted channel to request a response from the 182 client. The default is 0, indicating that these messages will 183 not be sent to the client. This option applies to protocol 184 version 2 only. 185 186 Compression 187 Specifies whether compression is allowed, or delayed until the 188 user has authenticated successfully. The argument must be 189 ``yes'', ``delayed'', or ``no''. The default is ``delayed''. 190 191 DenyGroups 192 This keyword can be followed by a list of group name patterns, 193 separated by spaces. Login is disallowed for users whose primary 194 group or supplementary group list matches one of the patterns. 195 Only group names are valid; a numerical group ID is not 196 recognized. By default, login is allowed for all groups. The 197 allow/deny directives are processed in the following order: 198 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups. 199 200 See PATTERNS in ssh_config(5) for more information on patterns. 201 202 DenyUsers 203 This keyword can be followed by a list of user name patterns, 204 separated by spaces. Login is disallowed for user names that 205 match one of the patterns. Only user names are valid; a 206 numerical user ID is not recognized. By default, login is 207 allowed for all users. If the pattern takes the form USER@HOST 208 then USER and HOST are separately checked, restricting logins to 209 particular users from particular hosts. The allow/deny 210 directives are processed in the following order: DenyUsers, 211 AllowUsers, DenyGroups, and finally AllowGroups. 212 213 See PATTERNS in ssh_config(5) for more information on patterns. 214 215 ForceCommand 216 Forces the execution of the command specified by ForceCommand, 217 ignoring any command supplied by the client and ~/.ssh/rc if 218 present. The command is invoked by using the user's login shell 219 with the -c option. This applies to shell, command, or subsystem 220 execution. It is most useful inside a Match block. The command 221 originally supplied by the client is available in the 222 SSH_ORIGINAL_COMMAND environment variable. Specifying a command 223 of ``internal-sftp'' will force the use of an in-process sftp 224 server that requires no support files when used with 225 ChrootDirectory. 226 227 GatewayPorts 228 Specifies whether remote hosts are allowed to connect to ports 229 forwarded for the client. By default, sshd(8) binds remote port 230 forwardings to the loopback address. This prevents other remote 231 hosts from connecting to forwarded ports. GatewayPorts can be 232 used to specify that sshd should allow remote port forwardings to 233 bind to non-loopback addresses, thus allowing other hosts to 234 connect. The argument may be ``no'' to force remote port 235 forwardings to be available to the local host only, ``yes'' to 236 force remote port forwardings to bind to the wildcard address, or 237 ``clientspecified'' to allow the client to select the address to 238 which the forwarding is bound. The default is ``no''. 239 240 GSSAPIAuthentication 241 Specifies whether user authentication based on GSSAPI is allowed. 242 The default is ``no''. Note that this option applies to protocol 243 version 2 only. 244 245 GSSAPICleanupCredentials 246 Specifies whether to automatically destroy the user's credentials 247 cache on logout. The default is ``yes''. Note that this option 248 applies to protocol version 2 only. 249 250 HostbasedAuthentication 251 Specifies whether rhosts or /etc/hosts.equiv authentication 252 together with successful public key client host authentication is 253 allowed (host-based authentication). This option is similar to 254 RhostsRSAAuthentication and applies to protocol version 2 only. 255 The default is ``no''. 256 257 HostbasedUsesNameFromPacketOnly 258 Specifies whether or not the server will attempt to perform a 259 reverse name lookup when matching the name in the ~/.shosts, 260 ~/.rhosts, and /etc/hosts.equiv files during 261 HostbasedAuthentication. A setting of ``yes'' means that sshd(8) 262 uses the name supplied by the client rather than attempting to 263 resolve the name from the TCP connection itself. The default is 264 ``no''. 265 266 HostCertificate 267 Specifies a file containing a public host certificate. The 268 certificate's public key must match a private host key already 269 specified by HostKey. The default behaviour of sshd(8) is not to 270 load any certificates. 271 272 HostKey 273 Specifies a file containing a private host key used by SSH. The 274 default is /etc/ssh/ssh_host_key for protocol version 1, and 275 /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key and 276 /etc/ssh/ssh_host_rsa_key for protocol version 2. Note that 277 sshd(8) will refuse to use a file if it is group/world- 278 accessible. It is possible to have multiple host key files. 279 ``rsa1'' keys are used for version 1 and ``dsa'', ``ecdsa'' or 280 ``rsa'' are used for version 2 of the SSH protocol. 281 282 IgnoreRhosts 283 Specifies that .rhosts and .shosts files will not be used in 284 RhostsRSAAuthentication or HostbasedAuthentication. 285 286 /etc/hosts.equiv and /etc/shosts.equiv are still used. The 287 default is ``yes''. 288 289 IgnoreUserKnownHosts 290 Specifies whether sshd(8) should ignore the user's 291 ~/.ssh/known_hosts during RhostsRSAAuthentication or 292 HostbasedAuthentication. The default is ``no''. 293 294 IPQoS Specifies the IPv4 type-of-service or DSCP class for the 295 connection. Accepted values are ``af11'', ``af12'', ``af13'', 296 ``af14'', ``af22'', ``af23'', ``af31'', ``af32'', ``af33'', 297 ``af41'', ``af42'', ``af43'', ``cs0'', ``cs1'', ``cs2'', ``cs3'', 298 ``cs4'', ``cs5'', ``cs6'', ``cs7'', ``ef'', ``lowdelay'', 299 ``throughput'', ``reliability'', or a numeric value. This option 300 may take one or two arguments, separated by whitespace. If one 301 argument is specified, it is used as the packet class 302 unconditionally. If two values are specified, the first is 303 automatically selected for interactive sessions and the second 304 for non-interactive sessions. The default is ``lowdelay'' for 305 interactive sessions and ``throughput'' for non-interactive 306 sessions. 307 308 KerberosAuthentication 309 Specifies whether the password provided by the user for 310 PasswordAuthentication will be validated through the Kerberos 311 KDC. To use this option, the server needs a Kerberos servtab 312 which allows the verification of the KDC's identity. The default 313 is ``no''. 314 315 KerberosGetAFSToken 316 If AFS is active and the user has a Kerberos 5 TGT, attempt to 317 acquire an AFS token before accessing the user's home directory. 318 The default is ``no''. 319 320 KerberosOrLocalPasswd 321 If password authentication through Kerberos fails then the 322 password will be validated via any additional local mechanism 323 such as /etc/passwd. The default is ``yes''. 324 325 KerberosTicketCleanup 326 Specifies whether to automatically destroy the user's ticket 327 cache file on logout. The default is ``yes''. 328 329 KexAlgorithms 330 Specifies the available KEX (Key Exchange) algorithms. Multiple 331 algorithms must be comma-separated. The default is 332 ``ecdh-sha2-nistp256'', ``ecdh-sha2-nistp384'', 333 ``ecdh-sha2-nistp521'', ``diffie-hellman-group-exchange-sha256'', 334 ``diffie-hellman-group-exchange-sha1'', 335 ``diffie-hellman-group14-sha1'', ``diffie-hellman-group1-sha1''. 336 337 KeyRegenerationInterval 338 In protocol version 1, the ephemeral server key is automatically 339 regenerated after this many seconds (if it has been used). The 340 purpose of regeneration is to prevent decrypting captured 341 sessions by later breaking into the machine and stealing the 342 keys. The key is never stored anywhere. If the value is 0, the 343 key is never regenerated. The default is 3600 (seconds). 344 345 ListenAddress 346 Specifies the local addresses sshd(8) should listen on. The 347 following forms may be used: 348 349 ListenAddress host|IPv4_addr|IPv6_addr 350 ListenAddress host|IPv4_addr:port 351 ListenAddress [host|IPv6_addr]:port 352 353 If port is not specified, sshd will listen on the address and all 354 prior Port options specified. The default is to listen on all 355 local addresses. Multiple ListenAddress options are permitted. 356 Additionally, any Port options must precede this option for non- 357 port qualified addresses. 358 359 LoginGraceTime 360 The server disconnects after this time if the user has not 361 successfully logged in. If the value is 0, there is no time 362 limit. The default is 120 seconds. 363 364 LogLevel 365 Gives the verbosity level that is used when logging messages from 366 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO, 367 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO. 368 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify 369 higher levels of debugging output. Logging with a DEBUG level 370 violates the privacy of users and is not recommended. 371 372 MACs Specifies the available MAC (message authentication code) 373 algorithms. The MAC algorithm is used in protocol version 2 for 374 data integrity protection. Multiple algorithms must be comma- 375 separated. The default is: 376 377 hmac-md5,hmac-sha1,umac-64@openssh.com, 378 hmac-ripemd160,hmac-sha1-96,hmac-md5-96 379 380 Match Introduces a conditional block. If all of the criteria on the 381 Match line are satisfied, the keywords on the following lines 382 override those set in the global section of the config file, 383 until either another Match line or the end of the file. 384 385 The arguments to Match are one or more criteria-pattern pairs. 386 The available criteria are User, Group, Host, and Address. The 387 match patterns may consist of single entries or comma-separated 388 lists and may use the wildcard and negation operators described 389 in the PATTERNS section of ssh_config(5). 390 391 The patterns in an Address criteria may additionally contain 392 addresses to match in CIDR address/masklen format, e.g. 393 ``192.0.2.0/24'' or ``3ffe:ffff::/32''. Note that the mask 394 length provided must be consistent with the address - it is an 395 error to specify a mask length that is too long for the address 396 or one with bits set in this host portion of the address. For 397 example, ``192.0.2.0/33'' and ``192.0.2.0/8'' respectively. 398 399 Only a subset of keywords may be used on the lines following a 400 Match keyword. Available keywords are AllowAgentForwarding, 401 AllowTcpForwarding, AuthorizedKeysFile, AuthorizedPrincipalsFile, 402 Banner, ChrootDirectory, ForceCommand, GatewayPorts, 403 GSSAPIAuthentication, HostbasedAuthentication, 404 HostbasedUsesNameFromPacketOnly, KbdInteractiveAuthentication, 405 KerberosAuthentication, MaxAuthTries, MaxSessions, 406 PasswordAuthentication, PermitEmptyPasswords, PermitOpen, 407 PermitRootLogin, PermitTunnel, PubkeyAuthentication, 408 RhostsRSAAuthentication, RSAAuthentication, X11DisplayOffset, 409 X11Forwarding and X11UseLocalHost. 410 411 MaxAuthTries 412 Specifies the maximum number of authentication attempts permitted 413 per connection. Once the number of failures reaches half this 414 value, additional failures are logged. The default is 6. 415 416 MaxSessions 417 Specifies the maximum number of open sessions permitted per 418 network connection. The default is 10. 419 420 MaxStartups 421 Specifies the maximum number of concurrent unauthenticated 422 connections to the SSH daemon. Additional connections will be 423 dropped until authentication succeeds or the LoginGraceTime 424 expires for a connection. The default is 10. 425 426 Alternatively, random early drop can be enabled by specifying the 427 three colon separated values ``start:rate:full'' (e.g. 428 "10:30:60"). sshd(8) will refuse connection attempts with a 429 probability of ``rate/100'' (30%) if there are currently 430 ``start'' (10) unauthenticated connections. The probability 431 increases linearly and all connection attempts are refused if the 432 number of unauthenticated connections reaches ``full'' (60). 433 434 PasswordAuthentication 435 Specifies whether password authentication is allowed. The 436 default is ``yes''. 437 438 PermitEmptyPasswords 439 When password authentication is allowed, it specifies whether the 440 server allows login to accounts with empty password strings. The 441 default is ``no''. 442 443 PermitOpen 444 Specifies the destinations to which TCP port forwarding is 445 permitted. The forwarding specification must be one of the 446 following forms: 447 448 PermitOpen host:port 449 PermitOpen IPv4_addr:port 450 PermitOpen [IPv6_addr]:port 451 452 Multiple forwards may be specified by separating them with 453 whitespace. An argument of ``any'' can be used to remove all 454 restrictions and permit any forwarding requests. By default all 455 port forwarding requests are permitted. 456 457 PermitRootLogin 458 Specifies whether root can log in using ssh(1). The argument 459 must be ``yes'', ``without-password'', ``forced-commands-only'', 460 or ``no''. The default is ``yes''. 461 462 If this option is set to ``without-password'', password 463 authentication is disabled for root. 464 465 If this option is set to ``forced-commands-only'', root login 466 with public key authentication will be allowed, but only if the 467 command option has been specified (which may be useful for taking 468 remote backups even if root login is normally not allowed). All 469 other authentication methods are disabled for root. 470 471 If this option is set to ``no'', root is not allowed to log in. 472 473 PermitTunnel 474 Specifies whether tun(4) device forwarding is allowed. The 475 argument must be ``yes'', ``point-to-point'' (layer 3), 476 ``ethernet'' (layer 2), or ``no''. Specifying ``yes'' permits 477 both ``point-to-point'' and ``ethernet''. The default is ``no''. 478 479 PermitUserEnvironment 480 Specifies whether ~/.ssh/environment and environment= options in 481 ~/.ssh/authorized_keys are processed by sshd(8). The default is 482 ``no''. Enabling environment processing may enable users to 483 bypass access restrictions in some configurations using 484 mechanisms such as LD_PRELOAD. 485 486 PidFile 487 Specifies the file that contains the process ID of the SSH 488 daemon. The default is /var/run/sshd.pid. 489 490 Port Specifies the port number that sshd(8) listens on. The default 491 is 22. Multiple options of this type are permitted. See also 492 ListenAddress. 493 494 PrintLastLog 495 Specifies whether sshd(8) should print the date and time of the 496 last user login when a user logs in interactively. The default 497 is ``yes''. 498 499 PrintMotd 500 Specifies whether sshd(8) should print /etc/motd when a user logs 501 in interactively. (On some systems it is also printed by the 502 shell, /etc/profile, or equivalent.) The default is ``yes''. 503 504 Protocol 505 Specifies the protocol versions sshd(8) supports. The possible 506 values are `1' and `2'. Multiple versions must be comma- 507 separated. The default is `2'. Note that the order of the 508 protocol list does not indicate preference, because the client 509 selects among multiple protocol versions offered by the server. 510 Specifying ``2,1'' is identical to ``1,2''. 511 512 PubkeyAuthentication 513 Specifies whether public key authentication is allowed. The 514 default is ``yes''. Note that this option applies to protocol 515 version 2 only. 516 517 RevokedKeys 518 Specifies a list of revoked public keys. Keys listed in this 519 file will be refused for public key authentication. Note that if 520 this file is not readable, then public key authentication will be 521 refused for all users. 522 523 RhostsRSAAuthentication 524 Specifies whether rhosts or /etc/hosts.equiv authentication 525 together with successful RSA host authentication is allowed. The 526 default is ``no''. This option applies to protocol version 1 527 only. 528 529 RSAAuthentication 530 Specifies whether pure RSA authentication is allowed. The 531 default is ``yes''. This option applies to protocol version 1 532 only. 533 534 ServerKeyBits 535 Defines the number of bits in the ephemeral protocol version 1 536 server key. The minimum value is 512, and the default is 1024. 537 538 StrictModes 539 Specifies whether sshd(8) should check file modes and ownership 540 of the user's files and home directory before accepting login. 541 This is normally desirable because novices sometimes accidentally 542 leave their directory or files world-writable. The default is 543 ``yes''. Note that this does not apply to ChrootDirectory, whose 544 permissions and ownership are checked unconditionally. 545 546 Subsystem 547 Configures an external subsystem (e.g. file transfer daemon). 548 Arguments should be a subsystem name and a command (with optional 549 arguments) to execute upon subsystem request. 550 551 The command sftp-server(8) implements the ``sftp'' file transfer 552 subsystem. 553 554 Alternately the name ``internal-sftp'' implements an in-process 555 ``sftp'' server. This may simplify configurations using 556 ChrootDirectory to force a different filesystem root on clients. 557 558 By default no subsystems are defined. Note that this option 559 applies to protocol version 2 only. 560 561 SyslogFacility 562 Gives the facility code that is used when logging messages from 563 sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0, 564 LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The 565 default is AUTH. 566 567 TCPKeepAlive 568 Specifies whether the system should send TCP keepalive messages 569 to the other side. If they are sent, death of the connection or 570 crash of one of the machines will be properly noticed. However, 571 this means that connections will die if the route is down 572 temporarily, and some people find it annoying. On the other 573 hand, if TCP keepalives are not sent, sessions may hang 574 indefinitely on the server, leaving ``ghost'' users and consuming 575 server resources. 576 577 The default is ``yes'' (to send TCP keepalive messages), and the 578 server will notice if the network goes down or the client host 579 crashes. This avoids infinitely hanging sessions. 580 581 To disable TCP keepalive messages, the value should be set to 582 ``no''. 583 584 TrustedUserCAKeys 585 Specifies a file containing public keys of certificate 586 authorities that are trusted to sign user certificates for 587 authentication. Keys are listed one per line; empty lines and 588 comments starting with `#' are allowed. If a certificate is 589 presented for authentication and has its signing CA key listed in 590 this file, then it may be used for authentication for any user 591 listed in the certificate's principals list. Note that 592 certificates that lack a list of principals will not be permitted 593 for authentication using TrustedUserCAKeys. For more details on 594 certificates, see the CERTIFICATES section in ssh-keygen(1). 595 596 UseDNS Specifies whether sshd(8) should look up the remote host name and 597 check that the resolved host name for the remote IP address maps 598 back to the very same IP address. The default is ``yes''. 599 600 UseLogin 601 Specifies whether login(1) is used for interactive login 602 sessions. The default is ``no''. Note that login(1) is never 603 used for remote command execution. Note also, that if this is 604 enabled, X11Forwarding will be disabled because login(1) does not 605 know how to handle xauth(1) cookies. If UsePrivilegeSeparation 606 is specified, it will be disabled after authentication. 607 608 UsePAM Enables the Pluggable Authentication Module interface. If set to 609 ``yes'' this will enable PAM authentication using 610 ChallengeResponseAuthentication and PasswordAuthentication in 611 addition to PAM account and session module processing for all 612 authentication types. 613 614 Because PAM challenge-response authentication usually serves an 615 equivalent role to password authentication, you should disable 616 either PasswordAuthentication or ChallengeResponseAuthentication. 617 618 If UsePAM is enabled, you will not be able to run sshd(8) as a 619 non-root user. The default is ``no''. 620 621 UsePrivilegeSeparation 622 Specifies whether sshd(8) separates privileges by creating an 623 unprivileged child process to deal with incoming network traffic. 624 After successful authentication, another process will be created 625 that has the privilege of the authenticated user. The goal of 626 privilege separation is to prevent privilege escalation by 627 containing any corruption within the unprivileged processes. The 628 default is ``yes''. 629 630 X11DisplayOffset 631 Specifies the first display number available for sshd(8)'s X11 632 forwarding. This prevents sshd from interfering with real X11 633 servers. The default is 10. 634 635 X11Forwarding 636 Specifies whether X11 forwarding is permitted. The argument must 637 be ``yes'' or ``no''. The default is ``no''. 638 639 When X11 forwarding is enabled, there may be additional exposure 640 to the server and to client displays if the sshd(8) proxy display 641 is configured to listen on the wildcard address (see 642 X11UseLocalhost below), though this is not the default. 643 Additionally, the authentication spoofing and authentication data 644 verification and substitution occur on the client side. The 645 security risk of using X11 forwarding is that the client's X11 646 display server may be exposed to attack when the SSH client 647 requests forwarding (see the warnings for ForwardX11 in 648 ssh_config(5)). A system administrator may have a stance in 649 which they want to protect clients that may expose themselves to 650 attack by unwittingly requesting X11 forwarding, which can 651 warrant a ``no'' setting. 652 653 Note that disabling X11 forwarding does not prevent users from 654 forwarding X11 traffic, as users can always install their own 655 forwarders. X11 forwarding is automatically disabled if UseLogin 656 is enabled. 657 658 X11UseLocalhost 659 Specifies whether sshd(8) should bind the X11 forwarding server 660 to the loopback address or to the wildcard address. By default, 661 sshd binds the forwarding server to the loopback address and sets 662 the hostname part of the DISPLAY environment variable to 663 ``localhost''. This prevents remote hosts from connecting to the 664 proxy display. However, some older X11 clients may not function 665 with this configuration. X11UseLocalhost may be set to ``no'' to 666 specify that the forwarding server should be bound to the 667 wildcard address. The argument must be ``yes'' or ``no''. The 668 default is ``yes''. 669 670 XAuthLocation 671 Specifies the full pathname of the xauth(1) program. The default 672 is /usr/X11R6/bin/xauth. 673 674TIME FORMATS 675 sshd(8) command-line arguments and configuration file options that 676 specify time may be expressed using a sequence of the form: 677 time[qualifier], where time is a positive integer value and qualifier is 678 one of the following: 679 680 <none> seconds 681 s | S seconds 682 m | M minutes 683 h | H hours 684 d | D days 685 w | W weeks 686 687 Each member of the sequence is added together to calculate the total time 688 value. 689 690 Time format examples: 691 692 600 600 seconds (10 minutes) 693 10m 10 minutes 694 1h30m 1 hour 30 minutes (90 minutes) 695 696FILES 697 /etc/ssh/sshd_config 698 Contains configuration data for sshd(8). This file should be 699 writable by root only, but it is recommended (though not 700 necessary) that it be world-readable. 701 702SEE ALSO 703 sshd(8) 704 705AUTHORS 706 OpenSSH is a derivative of the original and free ssh 1.2.12 release by 707 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 708 de Raadt and Dug Song removed many bugs, re-added newer features and 709 created OpenSSH. Markus Friedl contributed the support for SSH protocol 710 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support 711 for privilege separation. 712 713OpenBSD 4.8 December 8, 2010 OpenBSD 4.8 714