1 SSHD_CONFIG(5) OpenBSD Programmer's Manual SSHD_CONFIG(5) 2 3 NAME 4 sshd_config - OpenSSH SSH daemon configuration file 5 6 SYNOPSIS 7 /etc/ssh/sshd_config 8 9 DESCRIPTION 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. Multiple files may be 85 listed, separated by whitespace. The default is 86 ``.ssh/authorized_keys .ssh/authorized_keys2''. 87 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. 96 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. 104 105 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 112 details). 113 114 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. 118 119 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''. 123 124 ChrootDirectory 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. 130 131 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. 136 137 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). 146 147 The default is not to chroot(2). 148 149 Ciphers 150 Specifies the ciphers allowed for protocol version 2. Multiple 151 ciphers must be comma-separated. The supported ciphers are 152 ``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'', ``aes256-cbc'', 153 ``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'', ``arcfour128'', 154 ``arcfour256'', ``arcfour'', ``blowfish-cbc'', and 155 ``cast128-cbc''. The default is: 156 157 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 158 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 159 aes256-cbc,arcfour 160 161 ClientAliveCountMax 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. 173 174 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. 178 179 ClientAliveInterval 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 185 version 2 only. 186 187 Compression 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''. 191 192 DenyGroups 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. 200 201 See PATTERNS in ssh_config(5) for more information on patterns. 202 203 DenyUsers 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. 213 214 See PATTERNS in ssh_config(5) for more information on patterns. 215 216 ForceCommand 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 226 ChrootDirectory. 227 228 GatewayPorts 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''. 240 241 GSSAPIAuthentication 242 Specifies whether user authentication based on GSSAPI is allowed. 243 The default is ``no''. Note that this option applies to protocol 244 version 2 only. 245 246 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. 250 251 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''. 257 258 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 265 ``no''. 266 267 HostCertificate 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. 272 273 HostKey 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. 282 283 IgnoreRhosts 284 Specifies that .rhosts and .shosts files will not be used in 285 RhostsRSAAuthentication or HostbasedAuthentication. 286 287 /etc/hosts.equiv and /etc/shosts.equiv are still used. The 288 default is ``yes''. 289 290 IgnoreUserKnownHosts 291 Specifies whether sshd(8) should ignore the user's 292 ~/.ssh/known_hosts during RhostsRSAAuthentication or 293 HostbasedAuthentication. The default is ``no''. 294 295 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 307 sessions. 308 309 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 314 is ``no''. 315 316 KerberosGetAFSToken 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''. 320 321 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''. 325 326 KerberosTicketCleanup 327 Specifies whether to automatically destroy the user's ticket 328 cache file on logout. The default is ``yes''. 329 330 KexAlgorithms 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''. 337 338 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). 345 346 ListenAddress 347 Specifies the local addresses sshd(8) should listen on. The 348 following forms may be used: 349 350 ListenAddress host|IPv4_addr|IPv6_addr 351 ListenAddress host|IPv4_addr:port 352 ListenAddress [host|IPv6_addr]:port 353 354 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. 359 360 LoginGraceTime 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. 364 365 LogLevel 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. 372 373 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: 377 378 hmac-md5,hmac-sha1,umac-64 (a] openssh.com, 379 hmac-ripemd160,hmac-sha1-96,hmac-md5-96, 380 hmac-sha2-256,hmac-sha256-96,hmac-sha2-512, 381 hmac-sha2-512-96 382 383 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. 387 388 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). 393 394 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. 401 402 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. 413 414 MaxAuthTries 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. 418 419 MaxSessions 420 Specifies the maximum number of open sessions permitted per 421 network connection. The default is 10. 422 423 MaxStartups 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. 428 429 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). 436 437 PasswordAuthentication 438 Specifies whether password authentication is allowed. The 439 default is ``yes''. 440 441 PermitEmptyPasswords 442 When password authentication is allowed, it specifies whether the 443 server allows login to accounts with empty password strings. The 444 default is ``no''. 445 446 PermitOpen 447 Specifies the destinations to which TCP port forwarding is 448 permitted. The forwarding specification must be one of the 449 following forms: 450 451 PermitOpen host:port 452 PermitOpen IPv4_addr:port 453 PermitOpen [IPv6_addr]:port 454 455 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. 459 460 PermitRootLogin 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''. 464 465 If this option is set to ``without-password'', password 466 authentication is disabled for root. 467 468 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. 473 474 If this option is set to ``no'', root is not allowed to log in. 475 476 PermitTunnel 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''. 481 482 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. 488 489 PidFile 490 Specifies the file that contains the process ID of the SSH 491 daemon. The default is /var/run/sshd.pid. 492 493 Port Specifies the port number that sshd(8) listens on. The default 494 is 22. Multiple options of this type are permitted. See also 495 ListenAddress. 496 497 PrintLastLog 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 500 is ``yes''. 501 502 PrintMotd 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''. 506 507 Protocol 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''. 514 515 PubkeyAuthentication 516 Specifies whether public key authentication is allowed. The 517 default is ``yes''. Note that this option applies to protocol 518 version 2 only. 519 520 RevokedKeys 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. 525 526 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 1 530 only. 531 532 RSAAuthentication 533 Specifies whether pure RSA authentication is allowed. The 534 default is ``yes''. This option applies to protocol version 1 535 only. 536 537 ServerKeyBits 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. 540 541 StrictModes 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. 548 549 Subsystem 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. 553 554 The command sftp-server(8) implements the ``sftp'' file transfer 555 subsystem. 556 557 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. 560 561 By default no subsystems are defined. Note that this option 562 applies to protocol version 2 only. 563 564 SyslogFacility 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 568 default is AUTH. 569 570 TCPKeepAlive 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 578 server resources. 579 580 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. 583 584 To disable TCP keepalive messages, the value should be set to 585 ``no''. 586 587 TrustedUserCAKeys 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). 598 599 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''. 602 603 UseLogin 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. 610 611 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. 616 617 Because PAM challenge-response authentication usually serves an 618 equivalent role to password authentication, you should disable 619 either PasswordAuthentication or ChallengeResponseAuthentication. 620 621 If UsePAM is enabled, you will not be able to run sshd(8) as a 622 non-root user. The default is ``no''. 623 624 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. 634 635 X11DisplayOffset 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. 639 640 X11Forwarding 641 Specifies whether X11 forwarding is permitted. The argument must 642 be ``yes'' or ``no''. The default is ``no''. 643 644 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. 657 658 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 661 is enabled. 662 663 X11UseLocalhost 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 673 default is ``yes''. 674 675 XAuthLocation 676 Specifies the full pathname of the xauth(1) program. The default 677 is /usr/X11R6/bin/xauth. 678 679 TIME FORMATS 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: 684 685 <none> seconds 686 s | S seconds 687 m | M minutes 688 h | H hours 689 d | D days 690 w | W weeks 691 692 Each member of the sequence is added together to calculate the total time 693 value. 694 695 Time format examples: 696 697 600 600 seconds (10 minutes) 698 10m 10 minutes 699 1h30m 1 hour 30 minutes (90 minutes) 700 701 FILES 702 /etc/ssh/sshd_config 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. 706 707 SEE ALSO 708 sshd(8) 709 710 AUTHORS 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. 717 718 OpenBSD 5.0 August 2, 2011 OpenBSD 5.0 719
