sshd_config.0 revision 248613
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 available 57 options are ``yes'' or ``all'' to allow TCP forwarding, ``no'' to 58 prevent all TCP forwarding, ``local'' to allow local (from the 59 perspective of ssh(1)) forwarding only or ``remote'' to allow 60 remote forwarding only. The default is ``yes''. Note that 61 disabling TCP forwarding does not improve security unless users 62 are also denied shell access, as they can always install their 63 own forwarders. 64 65 AllowUsers 66 This keyword can be followed by a list of user name patterns, 67 separated by spaces. If specified, login is allowed only for 68 user names that match one of the patterns. Only user names are 69 valid; a numerical user ID is not recognized. By default, login 70 is allowed for all users. If the pattern takes the form 71 USER@HOST then USER and HOST are separately checked, restricting 72 logins to particular users from particular hosts. The allow/deny 73 directives are processed in the following order: DenyUsers, 74 AllowUsers, DenyGroups, and finally AllowGroups. 75 76 See PATTERNS in ssh_config(5) for more information on patterns. 77 78 AuthenticationMethods 79 Specifies the authentication methods that must be successfully 80 completed for a user to be granted access. This option must be 81 followed by one or more comma-separated lists of authentication 82 method names. Successful authentication requires completion of 83 every method in at least one of these lists. 84 85 For example, an argument of ``publickey,password 86 publickey,keyboard-interactive'' would require the user to 87 complete public key authentication, followed by either password 88 or keyboard interactive authentication. Only methods that are 89 next in one or more lists are offered at each stage, so for this 90 example, it would not be possible to attempt password or 91 keyboard-interactive authentication before public key. 92 93 This option is only available for SSH protocol 2 and will yield a 94 fatal error if enabled if protocol 1 is also enabled. Note that 95 each authentication method listed should also be explicitly 96 enabled in the configuration. The default is not to require 97 multiple authentication; successful completion of a single 98 authentication method is sufficient. 99 100 AuthorizedKeysCommand 101 Specifies a program to be used to look up the user's public keys. 102 The program will be invoked with a single argument of the 103 username being authenticated, and should produce on standard 104 output zero or more lines of authorized_keys output (see 105 AUTHORIZED_KEYS in sshd(8)). If a key supplied by 106 AuthorizedKeysCommand does not successfully authenticate and 107 authorize the user then public key authentication continues using 108 the usual AuthorizedKeysFile files. By default, no 109 AuthorizedKeysCommand is run. 110 111 AuthorizedKeysCommandUser 112 Specifies the user under whose account the AuthorizedKeysCommand 113 is run. It is recommended to use a dedicated user that has no 114 other role on the host than running authorized keys commands. 115 116 AuthorizedKeysFile 117 Specifies the file that contains the public keys that can be used 118 for user authentication. The format is described in the 119 AUTHORIZED_KEYS FILE FORMAT section of sshd(8). 120 AuthorizedKeysFile may contain tokens of the form %T which are 121 substituted during connection setup. The following tokens are 122 defined: %% is replaced by a literal '%', %h is replaced by the 123 home directory of the user being authenticated, and %u is 124 replaced by the username of that user. After expansion, 125 AuthorizedKeysFile is taken to be an absolute path or one 126 relative to the user's home directory. Multiple files may be 127 listed, separated by whitespace. The default is 128 ``.ssh/authorized_keys .ssh/authorized_keys2''. 129 130 AuthorizedPrincipalsFile 131 Specifies a file that lists principal names that are accepted for 132 certificate authentication. When using certificates signed by a 133 key listed in TrustedUserCAKeys, this file lists names, one of 134 which must appear in the certificate for it to be accepted for 135 authentication. Names are listed one per line preceded by key 136 options (as described in AUTHORIZED_KEYS FILE FORMAT in sshd(8)). 137 Empty lines and comments starting with `#' are ignored. 138 139 AuthorizedPrincipalsFile may contain tokens of the form %T which 140 are substituted during connection setup. The following tokens 141 are defined: %% is replaced by a literal '%', %h is replaced by 142 the home directory of the user being authenticated, and %u is 143 replaced by the username of that user. After expansion, 144 AuthorizedPrincipalsFile is taken to be an absolute path or one 145 relative to the user's home directory. 146 147 The default is ``none'', i.e. not to use a principals file - in 148 this case, the username of the user must appear in a 149 certificate's principals list for it to be accepted. Note that 150 AuthorizedPrincipalsFile is only used when authentication 151 proceeds using a CA listed in TrustedUserCAKeys and is not 152 consulted for certification authorities trusted via 153 ~/.ssh/authorized_keys, though the principals= key option offers 154 a similar facility (see sshd(8) for details). 155 156 Banner The contents of the specified file are sent to the remote user 157 before authentication is allowed. If the argument is ``none'' 158 then no banner is displayed. This option is only available for 159 protocol version 2. By default, no banner is displayed. 160 161 ChallengeResponseAuthentication 162 Specifies whether challenge-response authentication is allowed 163 (e.g. via PAM or though authentication styles supported in 164 login.conf(5)) The default is ``yes''. 165 166 ChrootDirectory 167 Specifies the pathname of a directory to chroot(2) to after 168 authentication. All components of the pathname must be root- 169 owned directories that are not writable by any other user or 170 group. After the chroot, sshd(8) changes the working directory 171 to the user's home directory. 172 173 The pathname may contain the following tokens that are expanded 174 at runtime once the connecting user has been authenticated: %% is 175 replaced by a literal '%', %h is replaced by the home directory 176 of the user being authenticated, and %u is replaced by the 177 username of that user. 178 179 The ChrootDirectory must contain the necessary files and 180 directories to support the user's session. For an interactive 181 session this requires at least a shell, typically sh(1), and 182 basic /dev nodes such as null(4), zero(4), stdin(4), stdout(4), 183 stderr(4), arandom(4) and tty(4) devices. For file transfer 184 sessions using ``sftp'', no additional configuration of the 185 environment is necessary if the in-process sftp server is used, 186 though sessions which use logging do require /dev/log inside the 187 chroot directory (see sftp-server(8) for details). 188 189 The default is not to chroot(2). 190 191 Ciphers 192 Specifies the ciphers allowed for protocol version 2. Multiple 193 ciphers must be comma-separated. The supported ciphers are 194 ``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'', ``aes256-cbc'', 195 ``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'', 196 ``aes128-gcm@openssh.com'', ``aes256-gcm@openssh.com'', 197 ``arcfour128'', ``arcfour256'', ``arcfour'', ``blowfish-cbc'', 198 and ``cast128-cbc''. The default is: 199 200 aes128-ctr,aes192-ctr,aes256-ctr,arcfour256,arcfour128, 201 aes128-gcm@openssh.com,aes256-gcm@openssh.com, 202 aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,aes192-cbc, 203 aes256-cbc,arcfour 204 205 ClientAliveCountMax 206 Sets the number of client alive messages (see below) which may be 207 sent without sshd(8) receiving any messages back from the client. 208 If this threshold is reached while client alive messages are 209 being sent, sshd will disconnect the client, terminating the 210 session. It is important to note that the use of client alive 211 messages is very different from TCPKeepAlive (below). The client 212 alive messages are sent through the encrypted channel and 213 therefore will not be spoofable. The TCP keepalive option 214 enabled by TCPKeepAlive is spoofable. The client alive mechanism 215 is valuable when the client or server depend on knowing when a 216 connection has become inactive. 217 218 The default value is 3. If ClientAliveInterval (see below) is 219 set to 15, and ClientAliveCountMax is left at the default, 220 unresponsive SSH clients will be disconnected after approximately 221 45 seconds. This option applies to protocol version 2 only. 222 223 ClientAliveInterval 224 Sets a timeout interval in seconds after which if no data has 225 been received from the client, sshd(8) will send a message 226 through the encrypted channel to request a response from the 227 client. The default is 0, indicating that these messages will 228 not be sent to the client. This option applies to protocol 229 version 2 only. 230 231 Compression 232 Specifies whether compression is allowed, or delayed until the 233 user has authenticated successfully. The argument must be 234 ``yes'', ``delayed'', or ``no''. The default is ``delayed''. 235 236 DenyGroups 237 This keyword can be followed by a list of group name patterns, 238 separated by spaces. Login is disallowed for users whose primary 239 group or supplementary group list matches one of the patterns. 240 Only group names are valid; a numerical group ID is not 241 recognized. By default, login is allowed for all groups. The 242 allow/deny directives are processed in the following order: 243 DenyUsers, AllowUsers, DenyGroups, and finally AllowGroups. 244 245 See PATTERNS in ssh_config(5) for more information on patterns. 246 247 DenyUsers 248 This keyword can be followed by a list of user name patterns, 249 separated by spaces. Login is disallowed for user names that 250 match one of the patterns. Only user names are valid; a 251 numerical user ID is not recognized. By default, login is 252 allowed for all users. If the pattern takes the form USER@HOST 253 then USER and HOST are separately checked, restricting logins to 254 particular users from particular hosts. The allow/deny 255 directives are processed in the following order: DenyUsers, 256 AllowUsers, DenyGroups, and finally AllowGroups. 257 258 See PATTERNS in ssh_config(5) for more information on patterns. 259 260 ForceCommand 261 Forces the execution of the command specified by ForceCommand, 262 ignoring any command supplied by the client and ~/.ssh/rc if 263 present. The command is invoked by using the user's login shell 264 with the -c option. This applies to shell, command, or subsystem 265 execution. It is most useful inside a Match block. The command 266 originally supplied by the client is available in the 267 SSH_ORIGINAL_COMMAND environment variable. Specifying a command 268 of ``internal-sftp'' will force the use of an in-process sftp 269 server that requires no support files when used with 270 ChrootDirectory. 271 272 GatewayPorts 273 Specifies whether remote hosts are allowed to connect to ports 274 forwarded for the client. By default, sshd(8) binds remote port 275 forwardings to the loopback address. This prevents other remote 276 hosts from connecting to forwarded ports. GatewayPorts can be 277 used to specify that sshd should allow remote port forwardings to 278 bind to non-loopback addresses, thus allowing other hosts to 279 connect. The argument may be ``no'' to force remote port 280 forwardings to be available to the local host only, ``yes'' to 281 force remote port forwardings to bind to the wildcard address, or 282 ``clientspecified'' to allow the client to select the address to 283 which the forwarding is bound. The default is ``no''. 284 285 GSSAPIAuthentication 286 Specifies whether user authentication based on GSSAPI is allowed. 287 The default is ``no''. Note that this option applies to protocol 288 version 2 only. 289 290 GSSAPICleanupCredentials 291 Specifies whether to automatically destroy the user's credentials 292 cache on logout. The default is ``yes''. Note that this option 293 applies to protocol version 2 only. 294 295 HostbasedAuthentication 296 Specifies whether rhosts or /etc/hosts.equiv authentication 297 together with successful public key client host authentication is 298 allowed (host-based authentication). This option is similar to 299 RhostsRSAAuthentication and applies to protocol version 2 only. 300 The default is ``no''. 301 302 HostbasedUsesNameFromPacketOnly 303 Specifies whether or not the server will attempt to perform a 304 reverse name lookup when matching the name in the ~/.shosts, 305 ~/.rhosts, and /etc/hosts.equiv files during 306 HostbasedAuthentication. A setting of ``yes'' means that sshd(8) 307 uses the name supplied by the client rather than attempting to 308 resolve the name from the TCP connection itself. The default is 309 ``no''. 310 311 HostCertificate 312 Specifies a file containing a public host certificate. The 313 certificate's public key must match a private host key already 314 specified by HostKey. The default behaviour of sshd(8) is not to 315 load any certificates. 316 317 HostKey 318 Specifies a file containing a private host key used by SSH. The 319 default is /etc/ssh/ssh_host_key for protocol version 1, and 320 /etc/ssh/ssh_host_dsa_key, /etc/ssh/ssh_host_ecdsa_key and 321 /etc/ssh/ssh_host_rsa_key for protocol version 2. Note that 322 sshd(8) will refuse to use a file if it is group/world- 323 accessible. It is possible to have multiple host key files. 324 ``rsa1'' keys are used for version 1 and ``dsa'', ``ecdsa'' or 325 ``rsa'' are used for version 2 of the SSH protocol. 326 327 IgnoreRhosts 328 Specifies that .rhosts and .shosts files will not be used in 329 RhostsRSAAuthentication or HostbasedAuthentication. 330 331 /etc/hosts.equiv and /etc/shosts.equiv are still used. The 332 default is ``yes''. 333 334 IgnoreUserKnownHosts 335 Specifies whether sshd(8) should ignore the user's 336 ~/.ssh/known_hosts during RhostsRSAAuthentication or 337 HostbasedAuthentication. The default is ``no''. 338 339 IPQoS Specifies the IPv4 type-of-service or DSCP class for the 340 connection. Accepted values are ``af11'', ``af12'', ``af13'', 341 ``af21'', ``af22'', ``af23'', ``af31'', ``af32'', ``af33'', 342 ``af41'', ``af42'', ``af43'', ``cs0'', ``cs1'', ``cs2'', ``cs3'', 343 ``cs4'', ``cs5'', ``cs6'', ``cs7'', ``ef'', ``lowdelay'', 344 ``throughput'', ``reliability'', or a numeric value. This option 345 may take one or two arguments, separated by whitespace. If one 346 argument is specified, it is used as the packet class 347 unconditionally. If two values are specified, the first is 348 automatically selected for interactive sessions and the second 349 for non-interactive sessions. The default is ``lowdelay'' for 350 interactive sessions and ``throughput'' for non-interactive 351 sessions. 352 353 KerberosAuthentication 354 Specifies whether the password provided by the user for 355 PasswordAuthentication will be validated through the Kerberos 356 KDC. To use this option, the server needs a Kerberos servtab 357 which allows the verification of the KDC's identity. The default 358 is ``no''. 359 360 KerberosGetAFSToken 361 If AFS is active and the user has a Kerberos 5 TGT, attempt to 362 acquire an AFS token before accessing the user's home directory. 363 The default is ``no''. 364 365 KerberosOrLocalPasswd 366 If password authentication through Kerberos fails then the 367 password will be validated via any additional local mechanism 368 such as /etc/passwd. The default is ``yes''. 369 370 KerberosTicketCleanup 371 Specifies whether to automatically destroy the user's ticket 372 cache file on logout. The default is ``yes''. 373 374 KexAlgorithms 375 Specifies the available KEX (Key Exchange) algorithms. Multiple 376 algorithms must be comma-separated. The default is 377 ``ecdh-sha2-nistp256'', ``ecdh-sha2-nistp384'', 378 ``ecdh-sha2-nistp521'', ``diffie-hellman-group-exchange-sha256'', 379 ``diffie-hellman-group-exchange-sha1'', 380 ``diffie-hellman-group14-sha1'', ``diffie-hellman-group1-sha1''. 381 382 KeyRegenerationInterval 383 In protocol version 1, the ephemeral server key is automatically 384 regenerated after this many seconds (if it has been used). The 385 purpose of regeneration is to prevent decrypting captured 386 sessions by later breaking into the machine and stealing the 387 keys. The key is never stored anywhere. If the value is 0, the 388 key is never regenerated. The default is 3600 (seconds). 389 390 ListenAddress 391 Specifies the local addresses sshd(8) should listen on. The 392 following forms may be used: 393 394 ListenAddress host|IPv4_addr|IPv6_addr 395 ListenAddress host|IPv4_addr:port 396 ListenAddress [host|IPv6_addr]:port 397 398 If port is not specified, sshd will listen on the address and all 399 prior Port options specified. The default is to listen on all 400 local addresses. Multiple ListenAddress options are permitted. 401 Additionally, any Port options must precede this option for non- 402 port qualified addresses. 403 404 LoginGraceTime 405 The server disconnects after this time if the user has not 406 successfully logged in. If the value is 0, there is no time 407 limit. The default is 120 seconds. 408 409 LogLevel 410 Gives the verbosity level that is used when logging messages from 411 sshd(8). The possible values are: QUIET, FATAL, ERROR, INFO, 412 VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. The default is INFO. 413 DEBUG and DEBUG1 are equivalent. DEBUG2 and DEBUG3 each specify 414 higher levels of debugging output. Logging with a DEBUG level 415 violates the privacy of users and is not recommended. 416 417 MACs Specifies the available MAC (message authentication code) 418 algorithms. The MAC algorithm is used in protocol version 2 for 419 data integrity protection. Multiple algorithms must be comma- 420 separated. The algorithms that contain ``-etm'' calculate the 421 MAC after encryption (encrypt-then-mac). These are considered 422 safer and their use recommended. The default is: 423 424 hmac-md5-etm@openssh.com,hmac-sha1-etm@openssh.com, 425 umac-64-etm@openssh.com,umac-128-etm@openssh.com, 426 hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, 427 hmac-ripemd160-etm@openssh.com,hmac-sha1-96-etm@openssh.com, 428 hmac-md5-96-etm@openssh.com, 429 hmac-md5,hmac-sha1,umac-64@openssh.com,umac-128@openssh.com, 430 hmac-sha2-256,hmac-sha2-512,hmac-ripemd160, 431 hmac-sha1-96,hmac-md5-96 432 433 Match Introduces a conditional block. If all of the criteria on the 434 Match line are satisfied, the keywords on the following lines 435 override those set in the global section of the config file, 436 until either another Match line or the end of the file. 437 438 The arguments to Match are one or more criteria-pattern pairs. 439 The available criteria are User, Group, Host, LocalAddress, 440 LocalPort, and Address. The match patterns may consist of single 441 entries or comma-separated lists and may use the wildcard and 442 negation operators described in the PATTERNS section of 443 ssh_config(5). 444 445 The patterns in an Address criteria may additionally contain 446 addresses to match in CIDR address/masklen format, e.g. 447 ``192.0.2.0/24'' or ``3ffe:ffff::/32''. Note that the mask 448 length provided must be consistent with the address - it is an 449 error to specify a mask length that is too long for the address 450 or one with bits set in this host portion of the address. For 451 example, ``192.0.2.0/33'' and ``192.0.2.0/8'' respectively. 452 453 Only a subset of keywords may be used on the lines following a 454 Match keyword. Available keywords are AcceptEnv, 455 AllowAgentForwarding, AllowGroups, AllowTcpForwarding, 456 AllowUsers, AuthenticationMethods, AuthorizedKeysCommand, 457 AuthorizedKeysCommandUser, AuthorizedKeysFile, 458 AuthorizedPrincipalsFile, Banner, ChrootDirectory, DenyGroups, 459 DenyUsers, ForceCommand, GatewayPorts, GSSAPIAuthentication, 460 HostbasedAuthentication, HostbasedUsesNameFromPacketOnly, 461 KbdInteractiveAuthentication, KerberosAuthentication, 462 MaxAuthTries, MaxSessions, PasswordAuthentication, 463 PermitEmptyPasswords, PermitOpen, PermitRootLogin, PermitTunnel, 464 PubkeyAuthentication, RhostsRSAAuthentication, RSAAuthentication, 465 X11DisplayOffset, X11Forwarding and X11UseLocalHost. 466 467 MaxAuthTries 468 Specifies the maximum number of authentication attempts permitted 469 per connection. Once the number of failures reaches half this 470 value, additional failures are logged. The default is 6. 471 472 MaxSessions 473 Specifies the maximum number of open sessions permitted per 474 network connection. The default is 10. 475 476 MaxStartups 477 Specifies the maximum number of concurrent unauthenticated 478 connections to the SSH daemon. Additional connections will be 479 dropped until authentication succeeds or the LoginGraceTime 480 expires for a connection. The default is 10:30:100. 481 482 Alternatively, random early drop can be enabled by specifying the 483 three colon separated values ``start:rate:full'' (e.g. 484 "10:30:60"). sshd(8) will refuse connection attempts with a 485 probability of ``rate/100'' (30%) if there are currently 486 ``start'' (10) unauthenticated connections. The probability 487 increases linearly and all connection attempts are refused if the 488 number of unauthenticated connections reaches ``full'' (60). 489 490 PasswordAuthentication 491 Specifies whether password authentication is allowed. The 492 default is ``yes''. 493 494 PermitEmptyPasswords 495 When password authentication is allowed, it specifies whether the 496 server allows login to accounts with empty password strings. The 497 default is ``no''. 498 499 PermitOpen 500 Specifies the destinations to which TCP port forwarding is 501 permitted. The forwarding specification must be one of the 502 following forms: 503 504 PermitOpen host:port 505 PermitOpen IPv4_addr:port 506 PermitOpen [IPv6_addr]:port 507 508 Multiple forwards may be specified by separating them with 509 whitespace. An argument of ``any'' can be used to remove all 510 restrictions and permit any forwarding requests. An argument of 511 ``none'' can be used to prohibit all forwarding requests. By 512 default all port forwarding requests are permitted. 513 514 PermitRootLogin 515 Specifies whether root can log in using ssh(1). The argument 516 must be ``yes'', ``without-password'', ``forced-commands-only'', 517 or ``no''. The default is ``yes''. 518 519 If this option is set to ``without-password'', password 520 authentication is disabled for root. 521 522 If this option is set to ``forced-commands-only'', root login 523 with public key authentication will be allowed, but only if the 524 command option has been specified (which may be useful for taking 525 remote backups even if root login is normally not allowed). All 526 other authentication methods are disabled for root. 527 528 If this option is set to ``no'', root is not allowed to log in. 529 530 PermitTunnel 531 Specifies whether tun(4) device forwarding is allowed. The 532 argument must be ``yes'', ``point-to-point'' (layer 3), 533 ``ethernet'' (layer 2), or ``no''. Specifying ``yes'' permits 534 both ``point-to-point'' and ``ethernet''. The default is ``no''. 535 536 PermitUserEnvironment 537 Specifies whether ~/.ssh/environment and environment= options in 538 ~/.ssh/authorized_keys are processed by sshd(8). The default is 539 ``no''. Enabling environment processing may enable users to 540 bypass access restrictions in some configurations using 541 mechanisms such as LD_PRELOAD. 542 543 PidFile 544 Specifies the file that contains the process ID of the SSH 545 daemon. The default is /var/run/sshd.pid. 546 547 Port Specifies the port number that sshd(8) listens on. The default 548 is 22. Multiple options of this type are permitted. See also 549 ListenAddress. 550 551 PrintLastLog 552 Specifies whether sshd(8) should print the date and time of the 553 last user login when a user logs in interactively. The default 554 is ``yes''. 555 556 PrintMotd 557 Specifies whether sshd(8) should print /etc/motd when a user logs 558 in interactively. (On some systems it is also printed by the 559 shell, /etc/profile, or equivalent.) The default is ``yes''. 560 561 Protocol 562 Specifies the protocol versions sshd(8) supports. The possible 563 values are `1' and `2'. Multiple versions must be comma- 564 separated. The default is `2'. Note that the order of the 565 protocol list does not indicate preference, because the client 566 selects among multiple protocol versions offered by the server. 567 Specifying ``2,1'' is identical to ``1,2''. 568 569 PubkeyAuthentication 570 Specifies whether public key authentication is allowed. The 571 default is ``yes''. Note that this option applies to protocol 572 version 2 only. 573 574 RevokedKeys 575 Specifies revoked public keys. Keys listed in this file will be 576 refused for public key authentication. Note that if this file is 577 not readable, then public key authentication will be refused for 578 all users. Keys may be specified as a text file, listing one 579 public key per line, or as an OpenSSH Key Revocation List (KRL) 580 as generated by ssh-keygen(1). For more information on KRLs, see 581 the KEY REVOCATION LISTS section in ssh-keygen(1). 582 583 RhostsRSAAuthentication 584 Specifies whether rhosts or /etc/hosts.equiv authentication 585 together with successful RSA host authentication is allowed. The 586 default is ``no''. This option applies to protocol version 1 587 only. 588 589 RSAAuthentication 590 Specifies whether pure RSA authentication is allowed. The 591 default is ``yes''. This option applies to protocol version 1 592 only. 593 594 ServerKeyBits 595 Defines the number of bits in the ephemeral protocol version 1 596 server key. The minimum value is 512, and the default is 1024. 597 598 StrictModes 599 Specifies whether sshd(8) should check file modes and ownership 600 of the user's files and home directory before accepting login. 601 This is normally desirable because novices sometimes accidentally 602 leave their directory or files world-writable. The default is 603 ``yes''. Note that this does not apply to ChrootDirectory, whose 604 permissions and ownership are checked unconditionally. 605 606 Subsystem 607 Configures an external subsystem (e.g. file transfer daemon). 608 Arguments should be a subsystem name and a command (with optional 609 arguments) to execute upon subsystem request. 610 611 The command sftp-server(8) implements the ``sftp'' file transfer 612 subsystem. 613 614 Alternately the name ``internal-sftp'' implements an in-process 615 ``sftp'' server. This may simplify configurations using 616 ChrootDirectory to force a different filesystem root on clients. 617 618 By default no subsystems are defined. Note that this option 619 applies to protocol version 2 only. 620 621 SyslogFacility 622 Gives the facility code that is used when logging messages from 623 sshd(8). The possible values are: DAEMON, USER, AUTH, LOCAL0, 624 LOCAL1, LOCAL2, LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The 625 default is AUTH. 626 627 TCPKeepAlive 628 Specifies whether the system should send TCP keepalive messages 629 to the other side. If they are sent, death of the connection or 630 crash of one of the machines will be properly noticed. However, 631 this means that connections will die if the route is down 632 temporarily, and some people find it annoying. On the other 633 hand, if TCP keepalives are not sent, sessions may hang 634 indefinitely on the server, leaving ``ghost'' users and consuming 635 server resources. 636 637 The default is ``yes'' (to send TCP keepalive messages), and the 638 server will notice if the network goes down or the client host 639 crashes. This avoids infinitely hanging sessions. 640 641 To disable TCP keepalive messages, the value should be set to 642 ``no''. 643 644 TrustedUserCAKeys 645 Specifies a file containing public keys of certificate 646 authorities that are trusted to sign user certificates for 647 authentication. Keys are listed one per line; empty lines and 648 comments starting with `#' are allowed. If a certificate is 649 presented for authentication and has its signing CA key listed in 650 this file, then it may be used for authentication for any user 651 listed in the certificate's principals list. Note that 652 certificates that lack a list of principals will not be permitted 653 for authentication using TrustedUserCAKeys. For more details on 654 certificates, see the CERTIFICATES section in ssh-keygen(1). 655 656 UseDNS Specifies whether sshd(8) should look up the remote host name and 657 check that the resolved host name for the remote IP address maps 658 back to the very same IP address. The default is ``yes''. 659 660 UseLogin 661 Specifies whether login(1) is used for interactive login 662 sessions. The default is ``no''. Note that login(1) is never 663 used for remote command execution. Note also, that if this is 664 enabled, X11Forwarding will be disabled because login(1) does not 665 know how to handle xauth(1) cookies. If UsePrivilegeSeparation 666 is specified, it will be disabled after authentication. 667 668 UsePAM Enables the Pluggable Authentication Module interface. If set to 669 ``yes'' this will enable PAM authentication using 670 ChallengeResponseAuthentication and PasswordAuthentication in 671 addition to PAM account and session module processing for all 672 authentication types. 673 674 Because PAM challenge-response authentication usually serves an 675 equivalent role to password authentication, you should disable 676 either PasswordAuthentication or ChallengeResponseAuthentication. 677 678 If UsePAM is enabled, you will not be able to run sshd(8) as a 679 non-root user. The default is ``no''. 680 681 UsePrivilegeSeparation 682 Specifies whether sshd(8) separates privileges by creating an 683 unprivileged child process to deal with incoming network traffic. 684 After successful authentication, another process will be created 685 that has the privilege of the authenticated user. The goal of 686 privilege separation is to prevent privilege escalation by 687 containing any corruption within the unprivileged processes. The 688 default is ``yes''. If UsePrivilegeSeparation is set to 689 ``sandbox'' then the pre-authentication unprivileged process is 690 subject to additional restrictions. 691 692 VersionAddendum 693 Optionally specifies additional text to append to the SSH 694 protocol banner sent by the server upon connection. The default 695 is ``none''. 696 697 X11DisplayOffset 698 Specifies the first display number available for sshd(8)'s X11 699 forwarding. This prevents sshd from interfering with real X11 700 servers. The default is 10. 701 702 X11Forwarding 703 Specifies whether X11 forwarding is permitted. The argument must 704 be ``yes'' or ``no''. The default is ``no''. 705 706 When X11 forwarding is enabled, there may be additional exposure 707 to the server and to client displays if the sshd(8) proxy display 708 is configured to listen on the wildcard address (see 709 X11UseLocalhost below), though this is not the default. 710 Additionally, the authentication spoofing and authentication data 711 verification and substitution occur on the client side. The 712 security risk of using X11 forwarding is that the client's X11 713 display server may be exposed to attack when the SSH client 714 requests forwarding (see the warnings for ForwardX11 in 715 ssh_config(5)). A system administrator may have a stance in 716 which they want to protect clients that may expose themselves to 717 attack by unwittingly requesting X11 forwarding, which can 718 warrant a ``no'' setting. 719 720 Note that disabling X11 forwarding does not prevent users from 721 forwarding X11 traffic, as users can always install their own 722 forwarders. X11 forwarding is automatically disabled if UseLogin 723 is enabled. 724 725 X11UseLocalhost 726 Specifies whether sshd(8) should bind the X11 forwarding server 727 to the loopback address or to the wildcard address. By default, 728 sshd binds the forwarding server to the loopback address and sets 729 the hostname part of the DISPLAY environment variable to 730 ``localhost''. This prevents remote hosts from connecting to the 731 proxy display. However, some older X11 clients may not function 732 with this configuration. X11UseLocalhost may be set to ``no'' to 733 specify that the forwarding server should be bound to the 734 wildcard address. The argument must be ``yes'' or ``no''. The 735 default is ``yes''. 736 737 XAuthLocation 738 Specifies the full pathname of the xauth(1) program. The default 739 is /usr/X11R6/bin/xauth. 740 741TIME FORMATS 742 sshd(8) command-line arguments and configuration file options that 743 specify time may be expressed using a sequence of the form: 744 time[qualifier], where time is a positive integer value and qualifier is 745 one of the following: 746 747 <none> seconds 748 s | S seconds 749 m | M minutes 750 h | H hours 751 d | D days 752 w | W weeks 753 754 Each member of the sequence is added together to calculate the total time 755 value. 756 757 Time format examples: 758 759 600 600 seconds (10 minutes) 760 10m 10 minutes 761 1h30m 1 hour 30 minutes (90 minutes) 762 763FILES 764 /etc/ssh/sshd_config 765 Contains configuration data for sshd(8). This file should be 766 writable by root only, but it is recommended (though not 767 necessary) that it be world-readable. 768 769SEE ALSO 770 sshd(8) 771 772AUTHORS 773 OpenSSH is a derivative of the original and free ssh 1.2.12 release by 774 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo 775 de Raadt and Dug Song removed many bugs, re-added newer features and 776 created OpenSSH. Markus Friedl contributed the support for SSH protocol 777 versions 1.5 and 2.0. Niels Provos and Markus Friedl contributed support 778 for privilege separation. 779 780OpenBSD 5.3 February 6, 2013 OpenBSD 5.3 781