Ben Lindstrom | 9f04903 | 2002-06-21 00:59:05 +0000 | [diff] [blame] | 1 | .\" -*- nroff -*- |
| 2 | .\" |
| 3 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> |
| 4 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland |
| 5 | .\" All rights reserved |
| 6 | .\" |
| 7 | .\" As far as I am concerned, the code I have written for this software |
| 8 | .\" can be used freely for any purpose. Any derived versions of this |
| 9 | .\" software must be clearly marked as such, and if the derived work is |
| 10 | .\" incompatible with the protocol description in the RFC file, it must be |
| 11 | .\" called by a name other than "ssh" or "Secure Shell". |
| 12 | .\" |
| 13 | .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. |
| 14 | .\" Copyright (c) 1999 Aaron Campbell. All rights reserved. |
| 15 | .\" Copyright (c) 1999 Theo de Raadt. All rights reserved. |
| 16 | .\" |
| 17 | .\" Redistribution and use in source and binary forms, with or without |
| 18 | .\" modification, are permitted provided that the following conditions |
| 19 | .\" are met: |
| 20 | .\" 1. Redistributions of source code must retain the above copyright |
| 21 | .\" notice, this list of conditions and the following disclaimer. |
| 22 | .\" 2. Redistributions in binary form must reproduce the above copyright |
| 23 | .\" notice, this list of conditions and the following disclaimer in the |
| 24 | .\" documentation and/or other materials provided with the distribution. |
| 25 | .\" |
| 26 | .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR |
| 27 | .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES |
| 28 | .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. |
| 29 | .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, |
| 30 | .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT |
| 31 | .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, |
| 32 | .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
| 33 | .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
| 34 | .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF |
| 35 | .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
| 36 | .\" |
Damien Miller | 5a93add | 2003-01-24 11:34:52 +1100 | [diff] [blame] | 37 | .\" $OpenBSD: sshd_config.5,v 1.14 2003/01/23 08:58:47 jmc Exp $ |
Ben Lindstrom | 9f04903 | 2002-06-21 00:59:05 +0000 | [diff] [blame] | 38 | .Dd September 25, 1999 |
| 39 | .Dt SSHD_CONFIG 5 |
| 40 | .Os |
| 41 | .Sh NAME |
| 42 | .Nm sshd_config |
| 43 | .Nd OpenSSH SSH daemon configuration file |
| 44 | .Sh SYNOPSIS |
| 45 | .Bl -tag -width Ds -compact |
| 46 | .It Pa /etc/ssh/sshd_config |
| 47 | .El |
| 48 | .Sh DESCRIPTION |
| 49 | .Nm sshd |
| 50 | reads configuration data from |
| 51 | .Pa /etc/ssh/sshd_config |
| 52 | (or the file specified with |
| 53 | .Fl f |
| 54 | on the command line). |
| 55 | The file contains keyword-argument pairs, one per line. |
| 56 | Lines starting with |
| 57 | .Ql # |
| 58 | and empty lines are interpreted as comments. |
| 59 | .Pp |
| 60 | The possible |
| 61 | keywords and their meanings are as follows (note that |
| 62 | keywords are case-insensitive and arguments are case-sensitive): |
| 63 | .Bl -tag -width Ds |
| 64 | .It Cm AFSTokenPassing |
| 65 | Specifies whether an AFS token may be forwarded to the server. |
| 66 | Default is |
| 67 | .Dq no . |
| 68 | .It Cm AllowGroups |
| 69 | This keyword can be followed by a list of group name patterns, separated |
| 70 | by spaces. |
| 71 | If specified, login is allowed only for users whose primary |
| 72 | group or supplementary group list matches one of the patterns. |
| 73 | .Ql \&* |
| 74 | and |
| 75 | .Ql ? |
| 76 | can be used as |
| 77 | wildcards in the patterns. |
| 78 | Only group names are valid; a numerical group ID is not recognized. |
| 79 | By default, login is allowed for all groups. |
| 80 | .Pp |
| 81 | .It Cm AllowTcpForwarding |
| 82 | Specifies whether TCP forwarding is permitted. |
| 83 | The default is |
| 84 | .Dq yes . |
| 85 | Note that disabling TCP forwarding does not improve security unless |
| 86 | users are also denied shell access, as they can always install their |
| 87 | own forwarders. |
| 88 | .Pp |
| 89 | .It Cm AllowUsers |
| 90 | This keyword can be followed by a list of user name patterns, separated |
| 91 | by spaces. |
Damien Miller | 5a93add | 2003-01-24 11:34:52 +1100 | [diff] [blame] | 92 | If specified, login is allowed only for user names that |
Ben Lindstrom | 9f04903 | 2002-06-21 00:59:05 +0000 | [diff] [blame] | 93 | match one of the patterns. |
| 94 | .Ql \&* |
| 95 | and |
| 96 | .Ql ? |
| 97 | can be used as |
| 98 | wildcards in the patterns. |
| 99 | Only user names are valid; a numerical user ID is not recognized. |
| 100 | By default, login is allowed for all users. |
| 101 | If the pattern takes the form USER@HOST then USER and HOST |
| 102 | are separately checked, restricting logins to particular |
| 103 | users from particular hosts. |
| 104 | .Pp |
| 105 | .It Cm AuthorizedKeysFile |
| 106 | Specifies the file that contains the public keys that can be used |
| 107 | for user authentication. |
| 108 | .Cm AuthorizedKeysFile |
| 109 | may contain tokens of the form %T which are substituted during connection |
| 110 | set-up. The following tokens are defined: %% is replaced by a literal '%', |
| 111 | %h is replaced by the home directory of the user being authenticated and |
| 112 | %u is replaced by the username of that user. |
| 113 | After expansion, |
| 114 | .Cm AuthorizedKeysFile |
| 115 | is taken to be an absolute path or one relative to the user's home |
| 116 | directory. |
| 117 | The default is |
| 118 | .Dq .ssh/authorized_keys . |
| 119 | .It Cm Banner |
| 120 | In some jurisdictions, sending a warning message before authentication |
| 121 | may be relevant for getting legal protection. |
| 122 | The contents of the specified file are sent to the remote user before |
| 123 | authentication is allowed. |
| 124 | This option is only available for protocol version 2. |
| 125 | By default, no banner is displayed. |
| 126 | .Pp |
| 127 | .It Cm ChallengeResponseAuthentication |
| 128 | Specifies whether challenge response authentication is allowed. |
| 129 | All authentication styles from |
| 130 | .Xr login.conf 5 |
| 131 | are supported. |
| 132 | The default is |
| 133 | .Dq yes . |
| 134 | .It Cm Ciphers |
| 135 | Specifies the ciphers allowed for protocol version 2. |
| 136 | Multiple ciphers must be comma-separated. |
| 137 | The default is |
| 138 | .Pp |
| 139 | .Bd -literal |
| 140 | ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour, |
| 141 | aes192-cbc,aes256-cbc'' |
| 142 | .Ed |
| 143 | .It Cm ClientAliveInterval |
| 144 | Sets a timeout interval in seconds after which if no data has been received |
| 145 | from the client, |
| 146 | .Nm sshd |
| 147 | will send a message through the encrypted |
| 148 | channel to request a response from the client. |
| 149 | The default |
| 150 | is 0, indicating that these messages will not be sent to the client. |
| 151 | This option applies to protocol version 2 only. |
| 152 | .It Cm ClientAliveCountMax |
| 153 | Sets the number of client alive messages (see above) which may be |
| 154 | sent without |
| 155 | .Nm sshd |
| 156 | receiving any messages back from the client. If this threshold is |
| 157 | reached while client alive messages are being sent, |
| 158 | .Nm sshd |
| 159 | will disconnect the client, terminating the session. It is important |
| 160 | to note that the use of client alive messages is very different from |
| 161 | .Cm KeepAlive |
| 162 | (below). The client alive messages are sent through the |
| 163 | encrypted channel and therefore will not be spoofable. The TCP keepalive |
| 164 | option enabled by |
| 165 | .Cm KeepAlive |
| 166 | is spoofable. The client alive mechanism is valuable when the client or |
| 167 | server depend on knowing when a connection has become inactive. |
| 168 | .Pp |
| 169 | The default value is 3. If |
| 170 | .Cm ClientAliveInterval |
| 171 | (above) is set to 15, and |
| 172 | .Cm ClientAliveCountMax |
| 173 | is left at the default, unresponsive ssh clients |
| 174 | will be disconnected after approximately 45 seconds. |
| 175 | .It Cm Compression |
| 176 | Specifies whether compression is allowed. |
| 177 | The argument must be |
| 178 | .Dq yes |
| 179 | or |
| 180 | .Dq no . |
| 181 | The default is |
| 182 | .Dq yes . |
| 183 | .It Cm DenyGroups |
| 184 | This keyword can be followed by a list of group name patterns, separated |
| 185 | by spaces. |
| 186 | Login is disallowed for users whose primary group or supplementary |
| 187 | group list matches one of the patterns. |
| 188 | .Ql \&* |
| 189 | and |
| 190 | .Ql ? |
| 191 | can be used as |
| 192 | wildcards in the patterns. |
| 193 | Only group names are valid; a numerical group ID is not recognized. |
| 194 | By default, login is allowed for all groups. |
| 195 | .Pp |
| 196 | .It Cm DenyUsers |
| 197 | This keyword can be followed by a list of user name patterns, separated |
| 198 | by spaces. |
| 199 | Login is disallowed for user names that match one of the patterns. |
| 200 | .Ql \&* |
| 201 | and |
| 202 | .Ql ? |
| 203 | can be used as wildcards in the patterns. |
| 204 | Only user names are valid; a numerical user ID is not recognized. |
| 205 | By default, login is allowed for all users. |
| 206 | If the pattern takes the form USER@HOST then USER and HOST |
| 207 | are separately checked, restricting logins to particular |
| 208 | users from particular hosts. |
| 209 | .It Cm GatewayPorts |
| 210 | Specifies whether remote hosts are allowed to connect to ports |
| 211 | forwarded for the client. |
| 212 | By default, |
| 213 | .Nm sshd |
| 214 | binds remote port forwardings to the loopback address. This |
| 215 | prevents other remote hosts from connecting to forwarded ports. |
| 216 | .Cm GatewayPorts |
| 217 | can be used to specify that |
| 218 | .Nm sshd |
| 219 | should bind remote port forwardings to the wildcard address, |
| 220 | thus allowing remote hosts to connect to forwarded ports. |
| 221 | The argument must be |
| 222 | .Dq yes |
| 223 | or |
| 224 | .Dq no . |
| 225 | The default is |
| 226 | .Dq no . |
| 227 | .It Cm HostbasedAuthentication |
| 228 | Specifies whether rhosts or /etc/hosts.equiv authentication together |
| 229 | with successful public key client host authentication is allowed |
| 230 | (hostbased authentication). |
| 231 | This option is similar to |
| 232 | .Cm RhostsRSAAuthentication |
| 233 | and applies to protocol version 2 only. |
| 234 | The default is |
| 235 | .Dq no . |
| 236 | .It Cm HostKey |
| 237 | Specifies a file containing a private host key |
| 238 | used by SSH. |
| 239 | The default is |
| 240 | .Pa /etc/ssh/ssh_host_key |
| 241 | for protocol version 1, and |
| 242 | .Pa /etc/ssh/ssh_host_rsa_key |
| 243 | and |
| 244 | .Pa /etc/ssh/ssh_host_dsa_key |
| 245 | for protocol version 2. |
| 246 | Note that |
| 247 | .Nm sshd |
| 248 | will refuse to use a file if it is group/world-accessible. |
| 249 | It is possible to have multiple host key files. |
| 250 | .Dq rsa1 |
| 251 | keys are used for version 1 and |
| 252 | .Dq dsa |
| 253 | or |
| 254 | .Dq rsa |
| 255 | are used for version 2 of the SSH protocol. |
| 256 | .It Cm IgnoreRhosts |
| 257 | Specifies that |
| 258 | .Pa .rhosts |
| 259 | and |
| 260 | .Pa .shosts |
| 261 | files will not be used in |
| 262 | .Cm RhostsAuthentication , |
| 263 | .Cm RhostsRSAAuthentication |
| 264 | or |
| 265 | .Cm HostbasedAuthentication . |
| 266 | .Pp |
| 267 | .Pa /etc/hosts.equiv |
| 268 | and |
| 269 | .Pa /etc/shosts.equiv |
| 270 | are still used. |
| 271 | The default is |
| 272 | .Dq yes . |
| 273 | .It Cm IgnoreUserKnownHosts |
| 274 | Specifies whether |
| 275 | .Nm sshd |
| 276 | should ignore the user's |
| 277 | .Pa $HOME/.ssh/known_hosts |
| 278 | during |
| 279 | .Cm RhostsRSAAuthentication |
| 280 | or |
| 281 | .Cm HostbasedAuthentication . |
| 282 | The default is |
| 283 | .Dq no . |
| 284 | .It Cm KeepAlive |
| 285 | Specifies whether the system should send TCP keepalive messages to the |
| 286 | other side. |
| 287 | If they are sent, death of the connection or crash of one |
| 288 | of the machines will be properly noticed. |
| 289 | However, this means that |
| 290 | connections will die if the route is down temporarily, and some people |
| 291 | find it annoying. |
| 292 | On the other hand, if keepalives are not sent, |
| 293 | sessions may hang indefinitely on the server, leaving |
| 294 | .Dq ghost |
| 295 | users and consuming server resources. |
| 296 | .Pp |
| 297 | The default is |
| 298 | .Dq yes |
| 299 | (to send keepalives), and the server will notice |
| 300 | if the network goes down or the client host crashes. |
| 301 | This avoids infinitely hanging sessions. |
| 302 | .Pp |
| 303 | To disable keepalives, the value should be set to |
| 304 | .Dq no . |
| 305 | .It Cm KerberosAuthentication |
| 306 | Specifies whether Kerberos authentication is allowed. |
| 307 | This can be in the form of a Kerberos ticket, or if |
Ben Lindstrom | 9f04903 | 2002-06-21 00:59:05 +0000 | [diff] [blame] | 308 | .Cm PasswordAuthentication |
| 309 | is yes, the password provided by the user will be validated through |
| 310 | the Kerberos KDC. |
| 311 | To use this option, the server needs a |
| 312 | Kerberos servtab which allows the verification of the KDC's identity. |
| 313 | Default is |
| 314 | .Dq no . |
| 315 | .It Cm KerberosOrLocalPasswd |
| 316 | If set then if password authentication through Kerberos fails then |
| 317 | the password will be validated via any additional local mechanism |
| 318 | such as |
| 319 | .Pa /etc/passwd . |
| 320 | Default is |
| 321 | .Dq yes . |
| 322 | .It Cm KerberosTgtPassing |
| 323 | Specifies whether a Kerberos TGT may be forwarded to the server. |
| 324 | Default is |
| 325 | .Dq no , |
| 326 | as this only works when the Kerberos KDC is actually an AFS kaserver. |
| 327 | .It Cm KerberosTicketCleanup |
| 328 | Specifies whether to automatically destroy the user's ticket cache |
| 329 | file on logout. |
| 330 | Default is |
| 331 | .Dq yes . |
| 332 | .It Cm KeyRegenerationInterval |
| 333 | In protocol version 1, the ephemeral server key is automatically regenerated |
| 334 | after this many seconds (if it has been used). |
| 335 | The purpose of regeneration is to prevent |
| 336 | decrypting captured sessions by later breaking into the machine and |
| 337 | stealing the keys. |
| 338 | The key is never stored anywhere. |
| 339 | If the value is 0, the key is never regenerated. |
| 340 | The default is 3600 (seconds). |
| 341 | .It Cm ListenAddress |
| 342 | Specifies the local addresses |
| 343 | .Nm sshd |
| 344 | should listen on. |
| 345 | The following forms may be used: |
| 346 | .Pp |
| 347 | .Bl -item -offset indent -compact |
| 348 | .It |
| 349 | .Cm ListenAddress |
| 350 | .Sm off |
| 351 | .Ar host No | Ar IPv4_addr No | Ar IPv6_addr |
| 352 | .Sm on |
| 353 | .It |
| 354 | .Cm ListenAddress |
| 355 | .Sm off |
| 356 | .Ar host No | Ar IPv4_addr No : Ar port |
| 357 | .Sm on |
| 358 | .It |
| 359 | .Cm ListenAddress |
| 360 | .Sm off |
| 361 | .Oo |
| 362 | .Ar host No | Ar IPv6_addr Oc : Ar port |
| 363 | .Sm on |
| 364 | .El |
| 365 | .Pp |
| 366 | If |
| 367 | .Ar port |
| 368 | is not specified, |
| 369 | .Nm sshd |
| 370 | will listen on the address and all prior |
| 371 | .Cm Port |
| 372 | options specified. The default is to listen on all local |
| 373 | addresses. Multiple |
| 374 | .Cm ListenAddress |
| 375 | options are permitted. Additionally, any |
| 376 | .Cm Port |
| 377 | options must precede this option for non port qualified addresses. |
| 378 | .It Cm LoginGraceTime |
| 379 | The server disconnects after this time if the user has not |
| 380 | successfully logged in. |
| 381 | If the value is 0, there is no time limit. |
Damien Miller | c134863 | 2002-09-05 14:35:14 +1000 | [diff] [blame] | 382 | The default is 120 seconds. |
Ben Lindstrom | 9f04903 | 2002-06-21 00:59:05 +0000 | [diff] [blame] | 383 | .It Cm LogLevel |
| 384 | Gives the verbosity level that is used when logging messages from |
| 385 | .Nm sshd . |
| 386 | The possible values are: |
| 387 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2 and DEBUG3. |
| 388 | The default is INFO. DEBUG and DEBUG1 are equivalent. DEBUG2 |
| 389 | and DEBUG3 each specify higher levels of debugging output. |
| 390 | Logging with a DEBUG level violates the privacy of users |
| 391 | and is not recommended. |
| 392 | .It Cm MACs |
| 393 | Specifies the available MAC (message authentication code) algorithms. |
| 394 | The MAC algorithm is used in protocol version 2 |
| 395 | for data integrity protection. |
| 396 | Multiple algorithms must be comma-separated. |
| 397 | The default is |
| 398 | .Dq hmac-md5,hmac-sha1,hmac-ripemd160,hmac-sha1-96,hmac-md5-96 . |
| 399 | .It Cm MaxStartups |
| 400 | Specifies the maximum number of concurrent unauthenticated connections to the |
| 401 | .Nm sshd |
| 402 | daemon. |
| 403 | Additional connections will be dropped until authentication succeeds or the |
| 404 | .Cm LoginGraceTime |
| 405 | expires for a connection. |
| 406 | The default is 10. |
| 407 | .Pp |
| 408 | Alternatively, random early drop can be enabled by specifying |
| 409 | the three colon separated values |
| 410 | .Dq start:rate:full |
| 411 | (e.g., "10:30:60"). |
| 412 | .Nm sshd |
| 413 | will refuse connection attempts with a probability of |
| 414 | .Dq rate/100 |
| 415 | (30%) |
| 416 | if there are currently |
| 417 | .Dq start |
| 418 | (10) |
| 419 | unauthenticated connections. |
| 420 | The probability increases linearly and all connection attempts |
| 421 | are refused if the number of unauthenticated connections reaches |
| 422 | .Dq full |
| 423 | (60). |
Damien Miller | 136d441 | 2002-06-26 23:05:16 +1000 | [diff] [blame] | 424 | .It Cm PAMAuthenticationViaKbdInt |
| 425 | Specifies whether PAM challenge response authentication is allowed. This |
| 426 | allows the use of most PAM challenge response authentication modules, but |
| 427 | it will allow password authentication regardless of whether |
| 428 | .Cm PasswordAuthentication |
| 429 | is enabled. |
Ben Lindstrom | 9f04903 | 2002-06-21 00:59:05 +0000 | [diff] [blame] | 430 | .It Cm PasswordAuthentication |
| 431 | Specifies whether password authentication is allowed. |
| 432 | The default is |
| 433 | .Dq yes . |
| 434 | .It Cm PermitEmptyPasswords |
| 435 | When password authentication is allowed, it specifies whether the |
| 436 | server allows login to accounts with empty password strings. |
| 437 | The default is |
| 438 | .Dq no . |
| 439 | .It Cm PermitRootLogin |
| 440 | Specifies whether root can login using |
| 441 | .Xr ssh 1 . |
| 442 | The argument must be |
| 443 | .Dq yes , |
| 444 | .Dq without-password , |
| 445 | .Dq forced-commands-only |
| 446 | or |
| 447 | .Dq no . |
| 448 | The default is |
| 449 | .Dq yes . |
| 450 | .Pp |
| 451 | If this option is set to |
| 452 | .Dq without-password |
| 453 | password authentication is disabled for root. |
| 454 | .Pp |
| 455 | If this option is set to |
| 456 | .Dq forced-commands-only |
| 457 | root login with public key authentication will be allowed, |
| 458 | but only if the |
| 459 | .Ar command |
| 460 | option has been specified |
| 461 | (which may be useful for taking remote backups even if root login is |
| 462 | normally not allowed). All other authentication methods are disabled |
| 463 | for root. |
| 464 | .Pp |
| 465 | If this option is set to |
| 466 | .Dq no |
| 467 | root is not allowed to login. |
Ben Lindstrom | 5d860f0 | 2002-08-01 01:28:38 +0000 | [diff] [blame] | 468 | .It Cm PermitUserEnvironment |
| 469 | Specifies whether |
| 470 | .Pa ~/.ssh/environment |
Ben Lindstrom | bd9bf38 | 2002-08-20 18:54:20 +0000 | [diff] [blame] | 471 | and |
Ben Lindstrom | 5d860f0 | 2002-08-01 01:28:38 +0000 | [diff] [blame] | 472 | .Cm environment= |
| 473 | options in |
| 474 | .Pa ~/.ssh/authorized_keys |
Ben Lindstrom | bd9bf38 | 2002-08-20 18:54:20 +0000 | [diff] [blame] | 475 | are processed by |
| 476 | .Nm sshd . |
Ben Lindstrom | 5d860f0 | 2002-08-01 01:28:38 +0000 | [diff] [blame] | 477 | The default is |
| 478 | .Dq no . |
Ben Lindstrom | bd9bf38 | 2002-08-20 18:54:20 +0000 | [diff] [blame] | 479 | Enabling environment processing may enable users to bypass access |
| 480 | restrictions in some configurations using mechanisms such as |
| 481 | .Ev LD_PRELOAD . |
Ben Lindstrom | 9f04903 | 2002-06-21 00:59:05 +0000 | [diff] [blame] | 482 | .It Cm PidFile |
Ben Lindstrom | 959de99 | 2002-06-23 00:35:25 +0000 | [diff] [blame] | 483 | Specifies the file that contains the process ID of the |
Ben Lindstrom | 9f04903 | 2002-06-21 00:59:05 +0000 | [diff] [blame] | 484 | .Nm sshd |
| 485 | daemon. |
| 486 | The default is |
| 487 | .Pa /var/run/sshd.pid . |
| 488 | .It Cm Port |
| 489 | Specifies the port number that |
| 490 | .Nm sshd |
| 491 | listens on. |
| 492 | The default is 22. |
| 493 | Multiple options of this type are permitted. |
| 494 | See also |
| 495 | .Cm ListenAddress . |
| 496 | .It Cm PrintLastLog |
| 497 | Specifies whether |
| 498 | .Nm sshd |
| 499 | should print the date and time when the user last logged in. |
| 500 | The default is |
| 501 | .Dq yes . |
| 502 | .It Cm PrintMotd |
| 503 | Specifies whether |
| 504 | .Nm sshd |
| 505 | should print |
| 506 | .Pa /etc/motd |
| 507 | when a user logs in interactively. |
| 508 | (On some systems it is also printed by the shell, |
| 509 | .Pa /etc/profile , |
| 510 | or equivalent.) |
| 511 | The default is |
| 512 | .Dq yes . |
| 513 | .It Cm Protocol |
| 514 | Specifies the protocol versions |
| 515 | .Nm sshd |
Ben Lindstrom | 9c44554 | 2002-07-11 03:59:18 +0000 | [diff] [blame] | 516 | supports. |
Ben Lindstrom | 9f04903 | 2002-06-21 00:59:05 +0000 | [diff] [blame] | 517 | The possible values are |
| 518 | .Dq 1 |
| 519 | and |
| 520 | .Dq 2 . |
| 521 | Multiple versions must be comma-separated. |
| 522 | The default is |
| 523 | .Dq 2,1 . |
Ben Lindstrom | 9c44554 | 2002-07-11 03:59:18 +0000 | [diff] [blame] | 524 | Note that the order of the protocol list does not indicate preference, |
| 525 | because the client selects among multiple protocol versions offered |
| 526 | by the server. |
| 527 | Specifying |
| 528 | .Dq 2,1 |
| 529 | is identical to |
| 530 | .Dq 1,2 . |
Ben Lindstrom | 9f04903 | 2002-06-21 00:59:05 +0000 | [diff] [blame] | 531 | .It Cm PubkeyAuthentication |
| 532 | Specifies whether public key authentication is allowed. |
| 533 | The default is |
| 534 | .Dq yes . |
| 535 | Note that this option applies to protocol version 2 only. |
| 536 | .It Cm RhostsAuthentication |
| 537 | Specifies whether authentication using rhosts or /etc/hosts.equiv |
| 538 | files is sufficient. |
| 539 | Normally, this method should not be permitted because it is insecure. |
| 540 | .Cm RhostsRSAAuthentication |
| 541 | should be used |
| 542 | instead, because it performs RSA-based host authentication in addition |
| 543 | to normal rhosts or /etc/hosts.equiv authentication. |
| 544 | The default is |
| 545 | .Dq no . |
| 546 | This option applies to protocol version 1 only. |
| 547 | .It Cm RhostsRSAAuthentication |
| 548 | Specifies whether rhosts or /etc/hosts.equiv authentication together |
| 549 | with successful RSA host authentication is allowed. |
| 550 | The default is |
| 551 | .Dq no . |
| 552 | This option applies to protocol version 1 only. |
| 553 | .It Cm RSAAuthentication |
| 554 | Specifies whether pure RSA authentication is allowed. |
| 555 | The default is |
| 556 | .Dq yes . |
| 557 | This option applies to protocol version 1 only. |
| 558 | .It Cm ServerKeyBits |
| 559 | Defines the number of bits in the ephemeral protocol version 1 server key. |
| 560 | The minimum value is 512, and the default is 768. |
| 561 | .It Cm StrictModes |
| 562 | Specifies whether |
| 563 | .Nm sshd |
| 564 | should check file modes and ownership of the |
| 565 | user's files and home directory before accepting login. |
| 566 | This is normally desirable because novices sometimes accidentally leave their |
| 567 | directory or files world-writable. |
| 568 | The default is |
| 569 | .Dq yes . |
| 570 | .It Cm Subsystem |
| 571 | Configures an external subsystem (e.g., file transfer daemon). |
| 572 | Arguments should be a subsystem name and a command to execute upon subsystem |
| 573 | request. |
| 574 | The command |
| 575 | .Xr sftp-server 8 |
| 576 | implements the |
| 577 | .Dq sftp |
| 578 | file transfer subsystem. |
| 579 | By default no subsystems are defined. |
| 580 | Note that this option applies to protocol version 2 only. |
| 581 | .It Cm SyslogFacility |
| 582 | Gives the facility code that is used when logging messages from |
| 583 | .Nm sshd . |
| 584 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, |
| 585 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. |
| 586 | The default is AUTH. |
| 587 | .It Cm UseLogin |
| 588 | Specifies whether |
| 589 | .Xr login 1 |
| 590 | is used for interactive login sessions. |
| 591 | The default is |
| 592 | .Dq no . |
| 593 | Note that |
| 594 | .Xr login 1 |
| 595 | is never used for remote command execution. |
| 596 | Note also, that if this is enabled, |
| 597 | .Cm X11Forwarding |
| 598 | will be disabled because |
| 599 | .Xr login 1 |
| 600 | does not know how to handle |
| 601 | .Xr xauth 1 |
| 602 | cookies. If |
| 603 | .Cm UsePrivilegeSeparation |
| 604 | is specified, it will be disabled after authentication. |
| 605 | .It Cm UsePrivilegeSeparation |
| 606 | Specifies whether |
| 607 | .Nm sshd |
| 608 | separates privileges by creating an unprivileged child process |
| 609 | to deal with incoming network traffic. After successful authentication, |
| 610 | another process will be created that has the privilege of the authenticated |
| 611 | user. The goal of privilege separation is to prevent privilege |
| 612 | escalation by containing any corruption within the unprivileged processes. |
| 613 | The default is |
| 614 | .Dq yes . |
| 615 | .It Cm VerifyReverseMapping |
| 616 | Specifies whether |
| 617 | .Nm sshd |
| 618 | should try to verify the remote host name and check that |
| 619 | the resolved host name for the remote IP address maps back to the |
| 620 | very same IP address. |
| 621 | The default is |
| 622 | .Dq no . |
| 623 | .It Cm X11DisplayOffset |
| 624 | Specifies the first display number available for |
| 625 | .Nm sshd Ns 's |
| 626 | X11 forwarding. |
| 627 | This prevents |
| 628 | .Nm sshd |
| 629 | from interfering with real X11 servers. |
| 630 | The default is 10. |
| 631 | .It Cm X11Forwarding |
| 632 | Specifies whether X11 forwarding is permitted. |
Damien Miller | 101c4a7 | 2002-09-19 11:51:21 +1000 | [diff] [blame] | 633 | The argument must be |
| 634 | .Dq yes |
| 635 | or |
| 636 | .Dq no . |
Ben Lindstrom | 9f04903 | 2002-06-21 00:59:05 +0000 | [diff] [blame] | 637 | The default is |
| 638 | .Dq no . |
Damien Miller | 101c4a7 | 2002-09-19 11:51:21 +1000 | [diff] [blame] | 639 | .Pp |
| 640 | When X11 forwarding is enabled, there may be additional exposure to |
| 641 | the server and to client displays if the |
| 642 | .Nm sshd |
| 643 | proxy display is configured to listen on the wildcard address (see |
| 644 | .Cm X11UseLocalhost |
| 645 | below), however this is not the default. |
| 646 | Additionally, the authentication spoofing and authentication data |
| 647 | verification and substitution occur on the client side. |
| 648 | The security risk of using X11 forwarding is that the client's X11 |
| 649 | display server may be exposed to attack when the ssh client requests |
| 650 | forwarding (see the warnings for |
| 651 | .Cm ForwardX11 |
| 652 | in |
| 653 | .Xr ssh_config 5 ). |
| 654 | A system administrator may have a stance in which they want to |
| 655 | protect clients that may expose themselves to attack by unwittingly |
| 656 | requesting X11 forwarding, which can warrant a |
| 657 | .Dq no |
| 658 | setting. |
| 659 | .Pp |
| 660 | Note that disabling X11 forwarding does not prevent users from |
| 661 | forwarding X11 traffic, as users can always install their own forwarders. |
Ben Lindstrom | 9f04903 | 2002-06-21 00:59:05 +0000 | [diff] [blame] | 662 | X11 forwarding is automatically disabled if |
| 663 | .Cm UseLogin |
| 664 | is enabled. |
| 665 | .It Cm X11UseLocalhost |
| 666 | Specifies whether |
| 667 | .Nm sshd |
| 668 | should bind the X11 forwarding server to the loopback address or to |
| 669 | the wildcard address. By default, |
| 670 | .Nm sshd |
| 671 | binds the forwarding server to the loopback address and sets the |
| 672 | hostname part of the |
| 673 | .Ev DISPLAY |
| 674 | environment variable to |
| 675 | .Dq localhost . |
Ben Lindstrom | 15b6120 | 2002-08-20 18:44:24 +0000 | [diff] [blame] | 676 | This prevents remote hosts from connecting to the proxy display. |
Ben Lindstrom | 9f04903 | 2002-06-21 00:59:05 +0000 | [diff] [blame] | 677 | However, some older X11 clients may not function with this |
| 678 | configuration. |
| 679 | .Cm X11UseLocalhost |
| 680 | may be set to |
| 681 | .Dq no |
| 682 | to specify that the forwarding server should be bound to the wildcard |
| 683 | address. |
| 684 | The argument must be |
| 685 | .Dq yes |
| 686 | or |
| 687 | .Dq no . |
| 688 | The default is |
| 689 | .Dq yes . |
| 690 | .It Cm XAuthLocation |
Damien Miller | 05913ba | 2002-09-04 16:51:03 +1000 | [diff] [blame] | 691 | Specifies the full pathname of the |
Ben Lindstrom | 9f04903 | 2002-06-21 00:59:05 +0000 | [diff] [blame] | 692 | .Xr xauth 1 |
| 693 | program. |
| 694 | The default is |
| 695 | .Pa /usr/X11R6/bin/xauth . |
| 696 | .El |
| 697 | .Ss Time Formats |
| 698 | .Pp |
| 699 | .Nm sshd |
| 700 | command-line arguments and configuration file options that specify time |
| 701 | may be expressed using a sequence of the form: |
| 702 | .Sm off |
Ben Lindstrom | 1f8cf4f | 2002-08-20 18:43:27 +0000 | [diff] [blame] | 703 | .Ar time Op Ar qualifier , |
Ben Lindstrom | 9f04903 | 2002-06-21 00:59:05 +0000 | [diff] [blame] | 704 | .Sm on |
| 705 | where |
| 706 | .Ar time |
| 707 | is a positive integer value and |
| 708 | .Ar qualifier |
| 709 | is one of the following: |
| 710 | .Pp |
| 711 | .Bl -tag -width Ds -compact -offset indent |
| 712 | .It Cm <none> |
| 713 | seconds |
| 714 | .It Cm s | Cm S |
| 715 | seconds |
| 716 | .It Cm m | Cm M |
| 717 | minutes |
| 718 | .It Cm h | Cm H |
| 719 | hours |
| 720 | .It Cm d | Cm D |
| 721 | days |
| 722 | .It Cm w | Cm W |
| 723 | weeks |
| 724 | .El |
| 725 | .Pp |
| 726 | Each member of the sequence is added together to calculate |
| 727 | the total time value. |
| 728 | .Pp |
| 729 | Time format examples: |
| 730 | .Pp |
| 731 | .Bl -tag -width Ds -compact -offset indent |
| 732 | .It 600 |
| 733 | 600 seconds (10 minutes) |
| 734 | .It 10m |
| 735 | 10 minutes |
| 736 | .It 1h30m |
| 737 | 1 hour 30 minutes (90 minutes) |
| 738 | .El |
| 739 | .Sh FILES |
| 740 | .Bl -tag -width Ds |
| 741 | .It Pa /etc/ssh/sshd_config |
| 742 | Contains configuration data for |
| 743 | .Nm sshd . |
| 744 | This file should be writable by root only, but it is recommended |
| 745 | (though not necessary) that it be world-readable. |
| 746 | .El |
| 747 | .Sh AUTHORS |
| 748 | OpenSSH is a derivative of the original and free |
| 749 | ssh 1.2.12 release by Tatu Ylonen. |
| 750 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, |
| 751 | Theo de Raadt and Dug Song |
| 752 | removed many bugs, re-added newer features and |
| 753 | created OpenSSH. |
| 754 | Markus Friedl contributed the support for SSH |
| 755 | protocol versions 1.5 and 2.0. |
| 756 | Niels Provos and Markus Friedl contributed support |
| 757 | for privilege separation. |
| 758 | .Sh SEE ALSO |
| 759 | .Xr sshd 8 |