Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1 | .\" |
| 2 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> |
| 3 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland |
| 4 | .\" All rights reserved |
| 5 | .\" |
| 6 | .\" As far as I am concerned, the code I have written for this software |
| 7 | .\" can be used freely for any purpose. Any derived versions of this |
| 8 | .\" software must be clearly marked as such, and if the derived work is |
| 9 | .\" incompatible with the protocol description in the RFC file, it must be |
| 10 | .\" called by a name other than "ssh" or "Secure Shell". |
| 11 | .\" |
| 12 | .\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. |
| 13 | .\" Copyright (c) 1999 Aaron Campbell. All rights reserved. |
| 14 | .\" Copyright (c) 1999 Theo de Raadt. All rights reserved. |
| 15 | .\" |
| 16 | .\" Redistribution and use in source and binary forms, with or without |
| 17 | .\" modification, are permitted provided that the following conditions |
| 18 | .\" are met: |
| 19 | .\" 1. Redistributions of source code must retain the above copyright |
| 20 | .\" notice, this list of conditions and the following disclaimer. |
| 21 | .\" 2. Redistributions in binary form must reproduce the above copyright |
| 22 | .\" notice, this list of conditions and the following disclaimer in the |
| 23 | .\" documentation and/or other materials provided with the distribution. |
| 24 | .\" |
| 25 | .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR |
| 26 | .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES |
| 27 | .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. |
| 28 | .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, |
| 29 | .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT |
| 30 | .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, |
| 31 | .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
| 32 | .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
| 33 | .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF |
| 34 | .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
| 35 | .\" |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 36 | .\" $OpenBSD: sshd_config.5,v 1.194 2015/02/20 23:46:01 djm Exp $ |
| 37 | .Dd $Mdocdate: February 20 2015 $ |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 38 | .Dt SSHD_CONFIG 5 |
| 39 | .Os |
| 40 | .Sh NAME |
| 41 | .Nm sshd_config |
| 42 | .Nd OpenSSH SSH daemon configuration file |
| 43 | .Sh SYNOPSIS |
| 44 | .Nm /etc/ssh/sshd_config |
| 45 | .Sh DESCRIPTION |
| 46 | .Xr sshd 8 |
| 47 | reads configuration data from |
| 48 | .Pa /etc/ssh/sshd_config |
| 49 | (or the file specified with |
| 50 | .Fl f |
| 51 | on the command line). |
| 52 | The file contains keyword-argument pairs, one per line. |
| 53 | Lines starting with |
| 54 | .Ql # |
| 55 | and empty lines are interpreted as comments. |
| 56 | Arguments may optionally be enclosed in double quotes |
| 57 | .Pq \&" |
| 58 | in order to represent arguments containing spaces. |
| 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 AcceptEnv |
| 65 | Specifies what environment variables sent by the client will be copied into |
| 66 | the session's |
| 67 | .Xr environ 7 . |
| 68 | See |
| 69 | .Cm SendEnv |
| 70 | in |
| 71 | .Xr ssh_config 5 |
| 72 | for how to configure the client. |
| 73 | Note that environment passing is only supported for protocol 2. |
| 74 | Variables are specified by name, which may contain the wildcard characters |
| 75 | .Ql * |
| 76 | and |
| 77 | .Ql \&? . |
| 78 | Multiple environment variables may be separated by whitespace or spread |
| 79 | across multiple |
| 80 | .Cm AcceptEnv |
| 81 | directives. |
| 82 | Be warned that some environment variables could be used to bypass restricted |
| 83 | user environments. |
| 84 | For this reason, care should be taken in the use of this directive. |
| 85 | The default is not to accept any environment variables. |
| 86 | .It Cm AddressFamily |
| 87 | Specifies which address family should be used by |
| 88 | .Xr sshd 8 . |
| 89 | Valid arguments are |
| 90 | .Dq any , |
| 91 | .Dq inet |
| 92 | (use IPv4 only), or |
| 93 | .Dq inet6 |
| 94 | (use IPv6 only). |
| 95 | The default is |
| 96 | .Dq any . |
| 97 | .It Cm AllowAgentForwarding |
| 98 | Specifies whether |
| 99 | .Xr ssh-agent 1 |
| 100 | forwarding is permitted. |
| 101 | The default is |
| 102 | .Dq yes . |
| 103 | Note that disabling agent forwarding does not improve security |
| 104 | unless users are also denied shell access, as they can always install |
| 105 | their own forwarders. |
| 106 | .It Cm AllowGroups |
| 107 | This keyword can be followed by a list of group name patterns, separated |
| 108 | by spaces. |
| 109 | If specified, login is allowed only for users whose primary |
| 110 | group or supplementary group list matches one of the patterns. |
| 111 | Only group names are valid; a numerical group ID is not recognized. |
| 112 | By default, login is allowed for all groups. |
| 113 | The allow/deny directives are processed in the following order: |
| 114 | .Cm DenyUsers , |
| 115 | .Cm AllowUsers , |
| 116 | .Cm DenyGroups , |
| 117 | and finally |
| 118 | .Cm AllowGroups . |
| 119 | .Pp |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 120 | See PATTERNS in |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 121 | .Xr ssh_config 5 |
| 122 | for more information on patterns. |
| 123 | .It Cm AllowTcpForwarding |
| 124 | Specifies whether TCP forwarding is permitted. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 125 | The available options are |
| 126 | .Dq yes |
| 127 | or |
| 128 | .Dq all |
| 129 | to allow TCP forwarding, |
| 130 | .Dq no |
| 131 | to prevent all TCP forwarding, |
| 132 | .Dq local |
| 133 | to allow local (from the perspective of |
| 134 | .Xr ssh 1 ) |
| 135 | forwarding only or |
| 136 | .Dq remote |
| 137 | to allow remote forwarding only. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 138 | The default is |
| 139 | .Dq yes . |
| 140 | Note that disabling TCP forwarding does not improve security unless |
| 141 | users are also denied shell access, as they can always install their |
| 142 | own forwarders. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 143 | .It Cm AllowStreamLocalForwarding |
| 144 | Specifies whether StreamLocal (Unix-domain socket) forwarding is permitted. |
| 145 | The available options are |
| 146 | .Dq yes |
| 147 | or |
| 148 | .Dq all |
| 149 | to allow StreamLocal forwarding, |
| 150 | .Dq no |
| 151 | to prevent all StreamLocal forwarding, |
| 152 | .Dq local |
| 153 | to allow local (from the perspective of |
| 154 | .Xr ssh 1 ) |
| 155 | forwarding only or |
| 156 | .Dq remote |
| 157 | to allow remote forwarding only. |
| 158 | The default is |
| 159 | .Dq yes . |
| 160 | Note that disabling StreamLocal forwarding does not improve security unless |
| 161 | users are also denied shell access, as they can always install their |
| 162 | own forwarders. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 163 | .It Cm AllowUsers |
| 164 | This keyword can be followed by a list of user name patterns, separated |
| 165 | by spaces. |
| 166 | If specified, login is allowed only for user names that |
| 167 | match one of the patterns. |
| 168 | Only user names are valid; a numerical user ID is not recognized. |
| 169 | By default, login is allowed for all users. |
| 170 | If the pattern takes the form USER@HOST then USER and HOST |
| 171 | are separately checked, restricting logins to particular |
| 172 | users from particular hosts. |
| 173 | The allow/deny directives are processed in the following order: |
| 174 | .Cm DenyUsers , |
| 175 | .Cm AllowUsers , |
| 176 | .Cm DenyGroups , |
| 177 | and finally |
| 178 | .Cm AllowGroups . |
| 179 | .Pp |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 180 | See PATTERNS in |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 181 | .Xr ssh_config 5 |
| 182 | for more information on patterns. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 183 | .It Cm AuthenticationMethods |
| 184 | Specifies the authentication methods that must be successfully completed |
| 185 | for a user to be granted access. |
| 186 | This option must be followed by one or more comma-separated lists of |
| 187 | authentication method names. |
| 188 | Successful authentication requires completion of every method in at least |
| 189 | one of these lists. |
| 190 | .Pp |
| 191 | For example, an argument of |
| 192 | .Dq publickey,password publickey,keyboard-interactive |
| 193 | would require the user to complete public key authentication, followed by |
| 194 | either password or keyboard interactive authentication. |
| 195 | Only methods that are next in one or more lists are offered at each stage, |
| 196 | so for this example, it would not be possible to attempt password or |
| 197 | keyboard-interactive authentication before public key. |
| 198 | .Pp |
| 199 | For keyboard interactive authentication it is also possible to |
| 200 | restrict authentication to a specific device by appending a |
| 201 | colon followed by the device identifier |
| 202 | .Dq bsdauth , |
| 203 | .Dq pam , |
| 204 | or |
| 205 | .Dq skey , |
| 206 | depending on the server configuration. |
| 207 | For example, |
| 208 | .Dq keyboard-interactive:bsdauth |
| 209 | would restrict keyboard interactive authentication to the |
| 210 | .Dq bsdauth |
| 211 | device. |
| 212 | .Pp |
| 213 | If the |
| 214 | .Dq publickey |
| 215 | method is listed more than once, |
| 216 | .Xr sshd 8 |
| 217 | verifies that keys that have been used successfully are not reused for |
| 218 | subsequent authentications. |
| 219 | For example, an |
| 220 | .Cm AuthenticationMethods |
| 221 | of |
| 222 | .Dq publickey,publickey |
| 223 | will require successful authentication using two different public keys. |
| 224 | .Pp |
| 225 | This option is only available for SSH protocol 2 and will yield a fatal |
| 226 | error if enabled if protocol 1 is also enabled. |
| 227 | Note that each authentication method listed should also be explicitly enabled |
| 228 | in the configuration. |
| 229 | The default is not to require multiple authentication; successful completion |
| 230 | of a single authentication method is sufficient. |
| 231 | .It Cm AuthorizedKeysCommand |
| 232 | Specifies a program to be used to look up the user's public keys. |
| 233 | The program must be owned by root and not writable by group or others. |
| 234 | It will be invoked with a single argument of the username |
| 235 | being authenticated, and should produce on standard output zero or |
| 236 | more lines of authorized_keys output (see AUTHORIZED_KEYS in |
| 237 | .Xr sshd 8 ) . |
| 238 | If a key supplied by AuthorizedKeysCommand does not successfully authenticate |
| 239 | and authorize the user then public key authentication continues using the usual |
| 240 | .Cm AuthorizedKeysFile |
| 241 | files. |
| 242 | By default, no AuthorizedKeysCommand is run. |
| 243 | .It Cm AuthorizedKeysCommandUser |
| 244 | Specifies the user under whose account the AuthorizedKeysCommand is run. |
| 245 | It is recommended to use a dedicated user that has no other role on the host |
| 246 | than running authorized keys commands. |
| 247 | If |
| 248 | .Cm AuthorizedKeysCommand |
| 249 | is specified but |
| 250 | .Cm AuthorizedKeysCommandUser |
| 251 | is not, then |
| 252 | .Xr sshd 8 |
| 253 | will refuse to start. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 254 | .It Cm AuthorizedKeysFile |
| 255 | Specifies the file that contains the public keys that can be used |
| 256 | for user authentication. |
| 257 | The format is described in the |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 258 | AUTHORIZED_KEYS FILE FORMAT |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 259 | section of |
| 260 | .Xr sshd 8 . |
| 261 | .Cm AuthorizedKeysFile |
| 262 | may contain tokens of the form %T which are substituted during connection |
| 263 | setup. |
| 264 | The following tokens are defined: %% is replaced by a literal '%', |
| 265 | %h is replaced by the home directory of the user being authenticated, and |
| 266 | %u is replaced by the username of that user. |
| 267 | After expansion, |
| 268 | .Cm AuthorizedKeysFile |
| 269 | is taken to be an absolute path or one relative to the user's home |
| 270 | directory. |
| 271 | Multiple files may be listed, separated by whitespace. |
| 272 | The default is |
| 273 | .Dq .ssh/authorized_keys .ssh/authorized_keys2 . |
| 274 | .It Cm AuthorizedPrincipalsFile |
| 275 | Specifies a file that lists principal names that are accepted for |
| 276 | certificate authentication. |
| 277 | When using certificates signed by a key listed in |
| 278 | .Cm TrustedUserCAKeys , |
| 279 | this file lists names, one of which must appear in the certificate for it |
| 280 | to be accepted for authentication. |
| 281 | Names are listed one per line preceded by key options (as described |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 282 | in AUTHORIZED_KEYS FILE FORMAT in |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 283 | .Xr sshd 8 ) . |
| 284 | Empty lines and comments starting with |
| 285 | .Ql # |
| 286 | are ignored. |
| 287 | .Pp |
| 288 | .Cm AuthorizedPrincipalsFile |
| 289 | may contain tokens of the form %T which are substituted during connection |
| 290 | setup. |
| 291 | The following tokens are defined: %% is replaced by a literal '%', |
| 292 | %h is replaced by the home directory of the user being authenticated, and |
| 293 | %u is replaced by the username of that user. |
| 294 | After expansion, |
| 295 | .Cm AuthorizedPrincipalsFile |
| 296 | is taken to be an absolute path or one relative to the user's home |
| 297 | directory. |
| 298 | .Pp |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 299 | The default is |
| 300 | .Dq none , |
| 301 | i.e. not to use a principals file \(en in this case, the username |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 302 | of the user must appear in a certificate's principals list for it to be |
| 303 | accepted. |
| 304 | Note that |
| 305 | .Cm AuthorizedPrincipalsFile |
| 306 | is only used when authentication proceeds using a CA listed in |
| 307 | .Cm TrustedUserCAKeys |
| 308 | and is not consulted for certification authorities trusted via |
| 309 | .Pa ~/.ssh/authorized_keys , |
| 310 | though the |
| 311 | .Cm principals= |
| 312 | key option offers a similar facility (see |
| 313 | .Xr sshd 8 |
| 314 | for details). |
| 315 | .It Cm Banner |
| 316 | The contents of the specified file are sent to the remote user before |
| 317 | authentication is allowed. |
| 318 | If the argument is |
| 319 | .Dq none |
| 320 | then no banner is displayed. |
| 321 | This option is only available for protocol version 2. |
| 322 | By default, no banner is displayed. |
| 323 | .It Cm ChallengeResponseAuthentication |
| 324 | Specifies whether challenge-response authentication is allowed (e.g. via |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 325 | PAM or through authentication styles supported in |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 326 | .Xr login.conf 5 ) |
| 327 | The default is |
| 328 | .Dq yes . |
| 329 | .It Cm ChrootDirectory |
| 330 | Specifies the pathname of a directory to |
| 331 | .Xr chroot 2 |
| 332 | to after authentication. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 333 | At session startup |
| 334 | .Xr sshd 8 |
| 335 | checks that all components of the pathname are root-owned directories |
| 336 | which are not writable by any other user or group. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 337 | After the chroot, |
| 338 | .Xr sshd 8 |
| 339 | changes the working directory to the user's home directory. |
| 340 | .Pp |
| 341 | The pathname may contain the following tokens that are expanded at runtime once |
| 342 | the connecting user has been authenticated: %% is replaced by a literal '%', |
| 343 | %h is replaced by the home directory of the user being authenticated, and |
| 344 | %u is replaced by the username of that user. |
| 345 | .Pp |
| 346 | The |
| 347 | .Cm ChrootDirectory |
| 348 | must contain the necessary files and directories to support the |
| 349 | user's session. |
| 350 | For an interactive session this requires at least a shell, typically |
| 351 | .Xr sh 1 , |
| 352 | and basic |
| 353 | .Pa /dev |
| 354 | nodes such as |
| 355 | .Xr null 4 , |
| 356 | .Xr zero 4 , |
| 357 | .Xr stdin 4 , |
| 358 | .Xr stdout 4 , |
| 359 | .Xr stderr 4 , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 360 | and |
| 361 | .Xr tty 4 |
| 362 | devices. |
| 363 | For file transfer sessions using |
| 364 | .Dq sftp , |
| 365 | no additional configuration of the environment is necessary if the |
| 366 | in-process sftp server is used, |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 367 | though sessions which use logging may require |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 368 | .Pa /dev/log |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 369 | inside the chroot directory on some operating systems (see |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 370 | .Xr sftp-server 8 |
| 371 | for details). |
| 372 | .Pp |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 373 | For safety, it is very important that the directory hierarchy be |
| 374 | prevented from modification by other processes on the system (especially |
| 375 | those outside the jail). |
| 376 | Misconfiguration can lead to unsafe environments which |
| 377 | .Xr sshd 8 |
| 378 | cannot detect. |
| 379 | .Pp |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 380 | The default is not to |
| 381 | .Xr chroot 2 . |
| 382 | .It Cm Ciphers |
| 383 | Specifies the ciphers allowed for protocol version 2. |
| 384 | Multiple ciphers must be comma-separated. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 385 | The supported ciphers are: |
| 386 | .Pp |
| 387 | .Bl -item -compact -offset indent |
| 388 | .It |
| 389 | 3des-cbc |
| 390 | .It |
| 391 | aes128-cbc |
| 392 | .It |
| 393 | aes192-cbc |
| 394 | .It |
| 395 | aes256-cbc |
| 396 | .It |
| 397 | aes128-ctr |
| 398 | .It |
| 399 | aes192-ctr |
| 400 | .It |
| 401 | aes256-ctr |
| 402 | .It |
| 403 | aes128-gcm@openssh.com |
| 404 | .It |
| 405 | aes256-gcm@openssh.com |
| 406 | .It |
| 407 | arcfour |
| 408 | .It |
| 409 | arcfour128 |
| 410 | .It |
| 411 | arcfour256 |
| 412 | .It |
| 413 | blowfish-cbc |
| 414 | .It |
| 415 | cast128-cbc |
| 416 | .It |
| 417 | chacha20-poly1305@openssh.com |
| 418 | .El |
| 419 | .Pp |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 420 | The default is: |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 421 | .Bd -literal -offset indent |
| 422 | aes128-ctr,aes192-ctr,aes256-ctr, |
| 423 | aes128-gcm@openssh.com,aes256-gcm@openssh.com, |
| 424 | chacha20-poly1305@openssh.com |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 425 | .Ed |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 426 | .Pp |
| 427 | The list of available ciphers may also be obtained using the |
| 428 | .Fl Q |
| 429 | option of |
| 430 | .Xr ssh 1 |
| 431 | with an argument of |
| 432 | .Dq cipher . |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 433 | .It Cm ClientAliveCountMax |
| 434 | Sets the number of client alive messages (see below) which may be |
| 435 | sent without |
| 436 | .Xr sshd 8 |
| 437 | receiving any messages back from the client. |
| 438 | If this threshold is reached while client alive messages are being sent, |
| 439 | sshd will disconnect the client, terminating the session. |
| 440 | It is important to note that the use of client alive messages is very |
| 441 | different from |
| 442 | .Cm TCPKeepAlive |
| 443 | (below). |
| 444 | The client alive messages are sent through the encrypted channel |
| 445 | and therefore will not be spoofable. |
| 446 | The TCP keepalive option enabled by |
| 447 | .Cm TCPKeepAlive |
| 448 | is spoofable. |
| 449 | The client alive mechanism is valuable when the client or |
| 450 | server depend on knowing when a connection has become inactive. |
| 451 | .Pp |
| 452 | The default value is 3. |
| 453 | If |
| 454 | .Cm ClientAliveInterval |
| 455 | (see below) is set to 15, and |
| 456 | .Cm ClientAliveCountMax |
| 457 | is left at the default, unresponsive SSH clients |
| 458 | will be disconnected after approximately 45 seconds. |
| 459 | This option applies to protocol version 2 only. |
| 460 | .It Cm ClientAliveInterval |
| 461 | Sets a timeout interval in seconds after which if no data has been received |
| 462 | from the client, |
| 463 | .Xr sshd 8 |
| 464 | will send a message through the encrypted |
| 465 | channel to request a response from the client. |
| 466 | The default |
| 467 | is 0, indicating that these messages will not be sent to the client. |
| 468 | This option applies to protocol version 2 only. |
| 469 | .It Cm Compression |
| 470 | Specifies whether compression is allowed, or delayed until |
| 471 | the user has authenticated successfully. |
| 472 | The argument must be |
| 473 | .Dq yes , |
| 474 | .Dq delayed , |
| 475 | or |
| 476 | .Dq no . |
| 477 | The default is |
| 478 | .Dq delayed . |
| 479 | .It Cm DenyGroups |
| 480 | This keyword can be followed by a list of group name patterns, separated |
| 481 | by spaces. |
| 482 | Login is disallowed for users whose primary group or supplementary |
| 483 | group list matches one of the patterns. |
| 484 | Only group names are valid; a numerical group ID is not recognized. |
| 485 | By default, login is allowed for all groups. |
| 486 | The allow/deny directives are processed in the following order: |
| 487 | .Cm DenyUsers , |
| 488 | .Cm AllowUsers , |
| 489 | .Cm DenyGroups , |
| 490 | and finally |
| 491 | .Cm AllowGroups . |
| 492 | .Pp |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 493 | See PATTERNS in |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 494 | .Xr ssh_config 5 |
| 495 | for more information on patterns. |
| 496 | .It Cm DenyUsers |
| 497 | This keyword can be followed by a list of user name patterns, separated |
| 498 | by spaces. |
| 499 | Login is disallowed for user names that match one of the patterns. |
| 500 | Only user names are valid; a numerical user ID is not recognized. |
| 501 | By default, login is allowed for all users. |
| 502 | If the pattern takes the form USER@HOST then USER and HOST |
| 503 | are separately checked, restricting logins to particular |
| 504 | users from particular hosts. |
| 505 | The allow/deny directives are processed in the following order: |
| 506 | .Cm DenyUsers , |
| 507 | .Cm AllowUsers , |
| 508 | .Cm DenyGroups , |
| 509 | and finally |
| 510 | .Cm AllowGroups . |
| 511 | .Pp |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 512 | See PATTERNS in |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 513 | .Xr ssh_config 5 |
| 514 | for more information on patterns. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 515 | .It Cm FingerprintHash |
| 516 | Specifies the hash algorithm used when logging key fingerprints. |
| 517 | Valid options are: |
| 518 | .Dq md5 |
| 519 | and |
| 520 | .Dq sha256 . |
| 521 | The default is |
| 522 | .Dq sha256 . |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 523 | .It Cm ForceCommand |
| 524 | Forces the execution of the command specified by |
| 525 | .Cm ForceCommand , |
| 526 | ignoring any command supplied by the client and |
| 527 | .Pa ~/.ssh/rc |
| 528 | if present. |
| 529 | The command is invoked by using the user's login shell with the -c option. |
| 530 | This applies to shell, command, or subsystem execution. |
| 531 | It is most useful inside a |
| 532 | .Cm Match |
| 533 | block. |
| 534 | The command originally supplied by the client is available in the |
| 535 | .Ev SSH_ORIGINAL_COMMAND |
| 536 | environment variable. |
| 537 | Specifying a command of |
| 538 | .Dq internal-sftp |
| 539 | will force the use of an in-process sftp server that requires no support |
| 540 | files when used with |
| 541 | .Cm ChrootDirectory . |
| 542 | .It Cm GatewayPorts |
| 543 | Specifies whether remote hosts are allowed to connect to ports |
| 544 | forwarded for the client. |
| 545 | By default, |
| 546 | .Xr sshd 8 |
| 547 | binds remote port forwardings to the loopback address. |
| 548 | This prevents other remote hosts from connecting to forwarded ports. |
| 549 | .Cm GatewayPorts |
| 550 | can be used to specify that sshd |
| 551 | should allow remote port forwardings to bind to non-loopback addresses, thus |
| 552 | allowing other hosts to connect. |
| 553 | The argument may be |
| 554 | .Dq no |
| 555 | to force remote port forwardings to be available to the local host only, |
| 556 | .Dq yes |
| 557 | to force remote port forwardings to bind to the wildcard address, or |
| 558 | .Dq clientspecified |
| 559 | to allow the client to select the address to which the forwarding is bound. |
| 560 | The default is |
| 561 | .Dq no . |
| 562 | .It Cm GSSAPIAuthentication |
| 563 | Specifies whether user authentication based on GSSAPI is allowed. |
| 564 | The default is |
| 565 | .Dq no . |
| 566 | Note that this option applies to protocol version 2 only. |
| 567 | .It Cm GSSAPICleanupCredentials |
| 568 | Specifies whether to automatically destroy the user's credentials cache |
| 569 | on logout. |
| 570 | The default is |
| 571 | .Dq yes . |
| 572 | Note that this option applies to protocol version 2 only. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 573 | .It Cm HostbasedAcceptedKeyTypes |
| 574 | Specifies the key types that will be accepted for hostbased authentication |
| 575 | as a comma-separated pattern list. |
| 576 | The default |
| 577 | .Dq * |
| 578 | will allow all key types. |
| 579 | The |
| 580 | .Fl Q |
| 581 | option of |
| 582 | .Xr ssh 1 |
| 583 | may be used to list supported key types. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 584 | .It Cm HostbasedAuthentication |
| 585 | Specifies whether rhosts or /etc/hosts.equiv authentication together |
| 586 | with successful public key client host authentication is allowed |
| 587 | (host-based authentication). |
| 588 | This option is similar to |
| 589 | .Cm RhostsRSAAuthentication |
| 590 | and applies to protocol version 2 only. |
| 591 | The default is |
| 592 | .Dq no . |
| 593 | .It Cm HostbasedUsesNameFromPacketOnly |
| 594 | Specifies whether or not the server will attempt to perform a reverse |
| 595 | name lookup when matching the name in the |
| 596 | .Pa ~/.shosts , |
| 597 | .Pa ~/.rhosts , |
| 598 | and |
| 599 | .Pa /etc/hosts.equiv |
| 600 | files during |
| 601 | .Cm HostbasedAuthentication . |
| 602 | A setting of |
| 603 | .Dq yes |
| 604 | means that |
| 605 | .Xr sshd 8 |
| 606 | uses the name supplied by the client rather than |
| 607 | attempting to resolve the name from the TCP connection itself. |
| 608 | The default is |
| 609 | .Dq no . |
| 610 | .It Cm HostCertificate |
| 611 | Specifies a file containing a public host certificate. |
| 612 | The certificate's public key must match a private host key already specified |
| 613 | by |
| 614 | .Cm HostKey . |
| 615 | The default behaviour of |
| 616 | .Xr sshd 8 |
| 617 | is not to load any certificates. |
| 618 | .It Cm HostKey |
| 619 | Specifies a file containing a private host key |
| 620 | used by SSH. |
| 621 | The default is |
| 622 | .Pa /etc/ssh/ssh_host_key |
| 623 | for protocol version 1, and |
| 624 | .Pa /etc/ssh/ssh_host_dsa_key , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 625 | .Pa /etc/ssh/ssh_host_ecdsa_key , |
| 626 | .Pa /etc/ssh/ssh_host_ed25519_key |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 627 | and |
| 628 | .Pa /etc/ssh/ssh_host_rsa_key |
| 629 | for protocol version 2. |
| 630 | Note that |
| 631 | .Xr sshd 8 |
| 632 | will refuse to use a file if it is group/world-accessible. |
| 633 | It is possible to have multiple host key files. |
| 634 | .Dq rsa1 |
| 635 | keys are used for version 1 and |
| 636 | .Dq dsa , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 637 | .Dq ecdsa , |
| 638 | .Dq ed25519 |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 639 | or |
| 640 | .Dq rsa |
| 641 | are used for version 2 of the SSH protocol. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 642 | It is also possible to specify public host key files instead. |
| 643 | In this case operations on the private key will be delegated |
| 644 | to an |
| 645 | .Xr ssh-agent 1 . |
| 646 | .It Cm HostKeyAgent |
| 647 | Identifies the UNIX-domain socket used to communicate |
| 648 | with an agent that has access to the private host keys. |
| 649 | If |
| 650 | .Dq SSH_AUTH_SOCK |
| 651 | is specified, the location of the socket will be read from the |
| 652 | .Ev SSH_AUTH_SOCK |
| 653 | environment variable. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 654 | .It Cm IgnoreRhosts |
| 655 | Specifies that |
| 656 | .Pa .rhosts |
| 657 | and |
| 658 | .Pa .shosts |
| 659 | files will not be used in |
| 660 | .Cm RhostsRSAAuthentication |
| 661 | or |
| 662 | .Cm HostbasedAuthentication . |
| 663 | .Pp |
| 664 | .Pa /etc/hosts.equiv |
| 665 | and |
| 666 | .Pa /etc/shosts.equiv |
| 667 | are still used. |
| 668 | The default is |
| 669 | .Dq yes . |
| 670 | .It Cm IgnoreUserKnownHosts |
| 671 | Specifies whether |
| 672 | .Xr sshd 8 |
| 673 | should ignore the user's |
| 674 | .Pa ~/.ssh/known_hosts |
| 675 | during |
| 676 | .Cm RhostsRSAAuthentication |
| 677 | or |
| 678 | .Cm HostbasedAuthentication . |
| 679 | The default is |
| 680 | .Dq no . |
| 681 | .It Cm IPQoS |
| 682 | Specifies the IPv4 type-of-service or DSCP class for the connection. |
| 683 | Accepted values are |
| 684 | .Dq af11 , |
| 685 | .Dq af12 , |
| 686 | .Dq af13 , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 687 | .Dq af21 , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 688 | .Dq af22 , |
| 689 | .Dq af23 , |
| 690 | .Dq af31 , |
| 691 | .Dq af32 , |
| 692 | .Dq af33 , |
| 693 | .Dq af41 , |
| 694 | .Dq af42 , |
| 695 | .Dq af43 , |
| 696 | .Dq cs0 , |
| 697 | .Dq cs1 , |
| 698 | .Dq cs2 , |
| 699 | .Dq cs3 , |
| 700 | .Dq cs4 , |
| 701 | .Dq cs5 , |
| 702 | .Dq cs6 , |
| 703 | .Dq cs7 , |
| 704 | .Dq ef , |
| 705 | .Dq lowdelay , |
| 706 | .Dq throughput , |
| 707 | .Dq reliability , |
| 708 | or a numeric value. |
| 709 | This option may take one or two arguments, separated by whitespace. |
| 710 | If one argument is specified, it is used as the packet class unconditionally. |
| 711 | If two values are specified, the first is automatically selected for |
| 712 | interactive sessions and the second for non-interactive sessions. |
| 713 | The default is |
| 714 | .Dq lowdelay |
| 715 | for interactive sessions and |
| 716 | .Dq throughput |
| 717 | for non-interactive sessions. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 718 | .It Cm KbdInteractiveAuthentication |
| 719 | Specifies whether to allow keyboard-interactive authentication. |
| 720 | The argument to this keyword must be |
| 721 | .Dq yes |
| 722 | or |
| 723 | .Dq no . |
| 724 | The default is to use whatever value |
| 725 | .Cm ChallengeResponseAuthentication |
| 726 | is set to |
| 727 | (by default |
| 728 | .Dq yes ) . |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 729 | .It Cm KerberosAuthentication |
| 730 | Specifies whether the password provided by the user for |
| 731 | .Cm PasswordAuthentication |
| 732 | will be validated through the Kerberos KDC. |
| 733 | To use this option, the server needs a |
| 734 | Kerberos servtab which allows the verification of the KDC's identity. |
| 735 | The default is |
| 736 | .Dq no . |
| 737 | .It Cm KerberosGetAFSToken |
| 738 | If AFS is active and the user has a Kerberos 5 TGT, attempt to acquire |
| 739 | an AFS token before accessing the user's home directory. |
| 740 | The default is |
| 741 | .Dq no . |
| 742 | .It Cm KerberosOrLocalPasswd |
| 743 | If password authentication through Kerberos fails then |
| 744 | the password will be validated via any additional local mechanism |
| 745 | such as |
| 746 | .Pa /etc/passwd . |
| 747 | The default is |
| 748 | .Dq yes . |
| 749 | .It Cm KerberosTicketCleanup |
| 750 | Specifies whether to automatically destroy the user's ticket cache |
| 751 | file on logout. |
| 752 | The default is |
| 753 | .Dq yes . |
| 754 | .It Cm KexAlgorithms |
| 755 | Specifies the available KEX (Key Exchange) algorithms. |
| 756 | Multiple algorithms must be comma-separated. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 757 | The supported algorithms are: |
| 758 | .Pp |
| 759 | .Bl -item -compact -offset indent |
| 760 | .It |
| 761 | curve25519-sha256@libssh.org |
| 762 | .It |
| 763 | diffie-hellman-group1-sha1 |
| 764 | .It |
| 765 | diffie-hellman-group14-sha1 |
| 766 | .It |
| 767 | diffie-hellman-group-exchange-sha1 |
| 768 | .It |
| 769 | diffie-hellman-group-exchange-sha256 |
| 770 | .It |
| 771 | ecdh-sha2-nistp256 |
| 772 | .It |
| 773 | ecdh-sha2-nistp384 |
| 774 | .It |
| 775 | ecdh-sha2-nistp521 |
| 776 | .El |
| 777 | .Pp |
| 778 | The default is: |
| 779 | .Bd -literal -offset indent |
| 780 | curve25519-sha256@libssh.org, |
| 781 | ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521, |
| 782 | diffie-hellman-group-exchange-sha256, |
| 783 | diffie-hellman-group14-sha1 |
| 784 | .Ed |
| 785 | .Pp |
| 786 | The list of available key exchange algorithms may also be obtained using the |
| 787 | .Fl Q |
| 788 | option of |
| 789 | .Xr ssh 1 |
| 790 | with an argument of |
| 791 | .Dq kex . |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 792 | .It Cm KeyRegenerationInterval |
| 793 | In protocol version 1, the ephemeral server key is automatically regenerated |
| 794 | after this many seconds (if it has been used). |
| 795 | The purpose of regeneration is to prevent |
| 796 | decrypting captured sessions by later breaking into the machine and |
| 797 | stealing the keys. |
| 798 | The key is never stored anywhere. |
| 799 | If the value is 0, the key is never regenerated. |
| 800 | The default is 3600 (seconds). |
| 801 | .It Cm ListenAddress |
| 802 | Specifies the local addresses |
| 803 | .Xr sshd 8 |
| 804 | should listen on. |
| 805 | The following forms may be used: |
| 806 | .Pp |
| 807 | .Bl -item -offset indent -compact |
| 808 | .It |
| 809 | .Cm ListenAddress |
| 810 | .Sm off |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 811 | .Ar host | Ar IPv4_addr | Ar IPv6_addr |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 812 | .Sm on |
| 813 | .It |
| 814 | .Cm ListenAddress |
| 815 | .Sm off |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 816 | .Ar host | Ar IPv4_addr : Ar port |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 817 | .Sm on |
| 818 | .It |
| 819 | .Cm ListenAddress |
| 820 | .Sm off |
| 821 | .Oo |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 822 | .Ar host | Ar IPv6_addr Oc : Ar port |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 823 | .Sm on |
| 824 | .El |
| 825 | .Pp |
| 826 | If |
| 827 | .Ar port |
| 828 | is not specified, |
| 829 | sshd will listen on the address and all prior |
| 830 | .Cm Port |
| 831 | options specified. |
| 832 | The default is to listen on all local addresses. |
| 833 | Multiple |
| 834 | .Cm ListenAddress |
| 835 | options are permitted. |
| 836 | Additionally, any |
| 837 | .Cm Port |
| 838 | options must precede this option for non-port qualified addresses. |
| 839 | .It Cm LoginGraceTime |
| 840 | The server disconnects after this time if the user has not |
| 841 | successfully logged in. |
| 842 | If the value is 0, there is no time limit. |
| 843 | The default is 120 seconds. |
| 844 | .It Cm LogLevel |
| 845 | Gives the verbosity level that is used when logging messages from |
| 846 | .Xr sshd 8 . |
| 847 | The possible values are: |
| 848 | QUIET, FATAL, ERROR, INFO, VERBOSE, DEBUG, DEBUG1, DEBUG2, and DEBUG3. |
| 849 | The default is INFO. |
| 850 | DEBUG and DEBUG1 are equivalent. |
| 851 | DEBUG2 and DEBUG3 each specify higher levels of debugging output. |
| 852 | Logging with a DEBUG level violates the privacy of users and is not recommended. |
| 853 | .It Cm MACs |
| 854 | Specifies the available MAC (message authentication code) algorithms. |
| 855 | The MAC algorithm is used in protocol version 2 |
| 856 | for data integrity protection. |
| 857 | Multiple algorithms must be comma-separated. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 858 | The algorithms that contain |
| 859 | .Dq -etm |
| 860 | calculate the MAC after encryption (encrypt-then-mac). |
| 861 | These are considered safer and their use recommended. |
| 862 | The supported MACs are: |
| 863 | .Pp |
| 864 | .Bl -item -compact -offset indent |
| 865 | .It |
| 866 | hmac-md5 |
| 867 | .It |
| 868 | hmac-md5-96 |
| 869 | .It |
| 870 | hmac-ripemd160 |
| 871 | .It |
| 872 | hmac-sha1 |
| 873 | .It |
| 874 | hmac-sha1-96 |
| 875 | .It |
| 876 | hmac-sha2-256 |
| 877 | .It |
| 878 | hmac-sha2-512 |
| 879 | .It |
| 880 | umac-64@openssh.com |
| 881 | .It |
| 882 | umac-128@openssh.com |
| 883 | .It |
| 884 | hmac-md5-etm@openssh.com |
| 885 | .It |
| 886 | hmac-md5-96-etm@openssh.com |
| 887 | .It |
| 888 | hmac-ripemd160-etm@openssh.com |
| 889 | .It |
| 890 | hmac-sha1-etm@openssh.com |
| 891 | .It |
| 892 | hmac-sha1-96-etm@openssh.com |
| 893 | .It |
| 894 | hmac-sha2-256-etm@openssh.com |
| 895 | .It |
| 896 | hmac-sha2-512-etm@openssh.com |
| 897 | .It |
| 898 | umac-64-etm@openssh.com |
| 899 | .It |
| 900 | umac-128-etm@openssh.com |
| 901 | .El |
| 902 | .Pp |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 903 | The default is: |
| 904 | .Bd -literal -offset indent |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 905 | umac-64-etm@openssh.com,umac-128-etm@openssh.com, |
| 906 | hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com, |
| 907 | umac-64@openssh.com,umac-128@openssh.com, |
| 908 | hmac-sha2-256,hmac-sha2-512 |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 909 | .Ed |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 910 | .Pp |
| 911 | The list of available MAC algorithms may also be obtained using the |
| 912 | .Fl Q |
| 913 | option of |
| 914 | .Xr ssh 1 |
| 915 | with an argument of |
| 916 | .Dq mac . |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 917 | .It Cm Match |
| 918 | Introduces a conditional block. |
| 919 | If all of the criteria on the |
| 920 | .Cm Match |
| 921 | line are satisfied, the keywords on the following lines override those |
| 922 | set in the global section of the config file, until either another |
| 923 | .Cm Match |
| 924 | line or the end of the file. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 925 | If a keyword appears in multiple |
| 926 | .Cm Match |
| 927 | blocks that are satisfied, only the first instance of the keyword is |
| 928 | applied. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 929 | .Pp |
| 930 | The arguments to |
| 931 | .Cm Match |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 932 | are one or more criteria-pattern pairs or the single token |
| 933 | .Cm All |
| 934 | which matches all criteria. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 935 | The available criteria are |
| 936 | .Cm User , |
| 937 | .Cm Group , |
| 938 | .Cm Host , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 939 | .Cm LocalAddress , |
| 940 | .Cm LocalPort , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 941 | and |
| 942 | .Cm Address . |
| 943 | The match patterns may consist of single entries or comma-separated |
| 944 | lists and may use the wildcard and negation operators described in the |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 945 | PATTERNS section of |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 946 | .Xr ssh_config 5 . |
| 947 | .Pp |
| 948 | The patterns in an |
| 949 | .Cm Address |
| 950 | criteria may additionally contain addresses to match in CIDR |
| 951 | address/masklen format, e.g.\& |
| 952 | .Dq 192.0.2.0/24 |
| 953 | or |
| 954 | .Dq 3ffe:ffff::/32 . |
| 955 | Note that the mask length provided must be consistent with the address - |
| 956 | it is an error to specify a mask length that is too long for the address |
| 957 | or one with bits set in this host portion of the address. |
| 958 | For example, |
| 959 | .Dq 192.0.2.0/33 |
| 960 | and |
| 961 | .Dq 192.0.2.0/8 |
| 962 | respectively. |
| 963 | .Pp |
| 964 | Only a subset of keywords may be used on the lines following a |
| 965 | .Cm Match |
| 966 | keyword. |
| 967 | Available keywords are |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 968 | .Cm AcceptEnv , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 969 | .Cm AllowAgentForwarding , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 970 | .Cm AllowGroups , |
| 971 | .Cm AllowStreamLocalForwarding , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 972 | .Cm AllowTcpForwarding , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 973 | .Cm AllowUsers , |
| 974 | .Cm AuthenticationMethods , |
| 975 | .Cm AuthorizedKeysCommand , |
| 976 | .Cm AuthorizedKeysCommandUser , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 977 | .Cm AuthorizedKeysFile , |
| 978 | .Cm AuthorizedPrincipalsFile , |
| 979 | .Cm Banner , |
| 980 | .Cm ChrootDirectory , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 981 | .Cm DenyGroups , |
| 982 | .Cm DenyUsers , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 983 | .Cm ForceCommand , |
| 984 | .Cm GatewayPorts , |
| 985 | .Cm GSSAPIAuthentication , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 986 | .Cm HostbasedAcceptedKeyTypes , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 987 | .Cm HostbasedAuthentication , |
| 988 | .Cm HostbasedUsesNameFromPacketOnly , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 989 | .Cm IPQoS , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 990 | .Cm KbdInteractiveAuthentication , |
| 991 | .Cm KerberosAuthentication , |
| 992 | .Cm MaxAuthTries , |
| 993 | .Cm MaxSessions , |
| 994 | .Cm PasswordAuthentication , |
| 995 | .Cm PermitEmptyPasswords , |
| 996 | .Cm PermitOpen , |
| 997 | .Cm PermitRootLogin , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 998 | .Cm PermitTTY , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 999 | .Cm PermitTunnel , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 1000 | .Cm PermitUserRC , |
| 1001 | .Cm PubkeyAcceptedKeyTypes , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1002 | .Cm PubkeyAuthentication , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 1003 | .Cm RekeyLimit , |
| 1004 | .Cm RevokedKeys , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1005 | .Cm RhostsRSAAuthentication , |
| 1006 | .Cm RSAAuthentication , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 1007 | .Cm StreamLocalBindMask , |
| 1008 | .Cm StreamLocalBindUnlink , |
| 1009 | .Cm TrustedUserCAKeys , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1010 | .Cm X11DisplayOffset , |
| 1011 | .Cm X11Forwarding |
| 1012 | and |
| 1013 | .Cm X11UseLocalHost . |
| 1014 | .It Cm MaxAuthTries |
| 1015 | Specifies the maximum number of authentication attempts permitted per |
| 1016 | connection. |
| 1017 | Once the number of failures reaches half this value, |
| 1018 | additional failures are logged. |
| 1019 | The default is 6. |
| 1020 | .It Cm MaxSessions |
| 1021 | Specifies the maximum number of open sessions permitted per network connection. |
| 1022 | The default is 10. |
| 1023 | .It Cm MaxStartups |
| 1024 | Specifies the maximum number of concurrent unauthenticated connections to the |
| 1025 | SSH daemon. |
| 1026 | Additional connections will be dropped until authentication succeeds or the |
| 1027 | .Cm LoginGraceTime |
| 1028 | expires for a connection. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 1029 | The default is 10:30:100. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1030 | .Pp |
| 1031 | Alternatively, random early drop can be enabled by specifying |
| 1032 | the three colon separated values |
| 1033 | .Dq start:rate:full |
| 1034 | (e.g. "10:30:60"). |
| 1035 | .Xr sshd 8 |
| 1036 | will refuse connection attempts with a probability of |
| 1037 | .Dq rate/100 |
| 1038 | (30%) |
| 1039 | if there are currently |
| 1040 | .Dq start |
| 1041 | (10) |
| 1042 | unauthenticated connections. |
| 1043 | The probability increases linearly and all connection attempts |
| 1044 | are refused if the number of unauthenticated connections reaches |
| 1045 | .Dq full |
| 1046 | (60). |
| 1047 | .It Cm PasswordAuthentication |
| 1048 | Specifies whether password authentication is allowed. |
| 1049 | The default is |
| 1050 | .Dq yes . |
| 1051 | .It Cm PermitEmptyPasswords |
| 1052 | When password authentication is allowed, it specifies whether the |
| 1053 | server allows login to accounts with empty password strings. |
| 1054 | The default is |
| 1055 | .Dq no . |
| 1056 | .It Cm PermitOpen |
| 1057 | Specifies the destinations to which TCP port forwarding is permitted. |
| 1058 | The forwarding specification must be one of the following forms: |
| 1059 | .Pp |
| 1060 | .Bl -item -offset indent -compact |
| 1061 | .It |
| 1062 | .Cm PermitOpen |
| 1063 | .Sm off |
| 1064 | .Ar host : port |
| 1065 | .Sm on |
| 1066 | .It |
| 1067 | .Cm PermitOpen |
| 1068 | .Sm off |
| 1069 | .Ar IPv4_addr : port |
| 1070 | .Sm on |
| 1071 | .It |
| 1072 | .Cm PermitOpen |
| 1073 | .Sm off |
| 1074 | .Ar \&[ IPv6_addr \&] : port |
| 1075 | .Sm on |
| 1076 | .El |
| 1077 | .Pp |
| 1078 | Multiple forwards may be specified by separating them with whitespace. |
| 1079 | An argument of |
| 1080 | .Dq any |
| 1081 | can be used to remove all restrictions and permit any forwarding requests. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 1082 | An argument of |
| 1083 | .Dq none |
| 1084 | can be used to prohibit all forwarding requests. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1085 | By default all port forwarding requests are permitted. |
| 1086 | .It Cm PermitRootLogin |
| 1087 | Specifies whether root can log in using |
| 1088 | .Xr ssh 1 . |
| 1089 | The argument must be |
| 1090 | .Dq yes , |
| 1091 | .Dq without-password , |
| 1092 | .Dq forced-commands-only , |
| 1093 | or |
| 1094 | .Dq no . |
| 1095 | The default is |
| 1096 | .Dq yes . |
| 1097 | .Pp |
| 1098 | If this option is set to |
| 1099 | .Dq without-password , |
| 1100 | password authentication is disabled for root. |
| 1101 | .Pp |
| 1102 | If this option is set to |
| 1103 | .Dq forced-commands-only , |
| 1104 | root login with public key authentication will be allowed, |
| 1105 | but only if the |
| 1106 | .Ar command |
| 1107 | option has been specified |
| 1108 | (which may be useful for taking remote backups even if root login is |
| 1109 | normally not allowed). |
| 1110 | All other authentication methods are disabled for root. |
| 1111 | .Pp |
| 1112 | If this option is set to |
| 1113 | .Dq no , |
| 1114 | root is not allowed to log in. |
| 1115 | .It Cm PermitTunnel |
| 1116 | Specifies whether |
| 1117 | .Xr tun 4 |
| 1118 | device forwarding is allowed. |
| 1119 | The argument must be |
| 1120 | .Dq yes , |
| 1121 | .Dq point-to-point |
| 1122 | (layer 3), |
| 1123 | .Dq ethernet |
| 1124 | (layer 2), or |
| 1125 | .Dq no . |
| 1126 | Specifying |
| 1127 | .Dq yes |
| 1128 | permits both |
| 1129 | .Dq point-to-point |
| 1130 | and |
| 1131 | .Dq ethernet . |
| 1132 | The default is |
| 1133 | .Dq no . |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 1134 | .Pp |
| 1135 | Independent of this setting, the permissions of the selected |
| 1136 | .Xr tun 4 |
| 1137 | device must allow access to the user. |
| 1138 | .It Cm PermitTTY |
| 1139 | Specifies whether |
| 1140 | .Xr pty 4 |
| 1141 | allocation is permitted. |
| 1142 | The default is |
| 1143 | .Dq yes . |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1144 | .It Cm PermitUserEnvironment |
| 1145 | Specifies whether |
| 1146 | .Pa ~/.ssh/environment |
| 1147 | and |
| 1148 | .Cm environment= |
| 1149 | options in |
| 1150 | .Pa ~/.ssh/authorized_keys |
| 1151 | are processed by |
| 1152 | .Xr sshd 8 . |
| 1153 | The default is |
| 1154 | .Dq no . |
| 1155 | Enabling environment processing may enable users to bypass access |
| 1156 | restrictions in some configurations using mechanisms such as |
| 1157 | .Ev LD_PRELOAD . |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 1158 | .It Cm PermitUserRC |
| 1159 | Specifies whether any |
| 1160 | .Pa ~/.ssh/rc |
| 1161 | file is executed. |
| 1162 | The default is |
| 1163 | .Dq yes . |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1164 | .It Cm PidFile |
| 1165 | Specifies the file that contains the process ID of the |
| 1166 | SSH daemon. |
| 1167 | The default is |
| 1168 | .Pa /var/run/sshd.pid . |
| 1169 | .It Cm Port |
| 1170 | Specifies the port number that |
| 1171 | .Xr sshd 8 |
| 1172 | listens on. |
| 1173 | The default is 22. |
| 1174 | Multiple options of this type are permitted. |
| 1175 | See also |
| 1176 | .Cm ListenAddress . |
| 1177 | .It Cm PrintLastLog |
| 1178 | Specifies whether |
| 1179 | .Xr sshd 8 |
| 1180 | should print the date and time of the last user login when a user logs |
| 1181 | in interactively. |
| 1182 | The default is |
| 1183 | .Dq yes . |
| 1184 | .It Cm PrintMotd |
| 1185 | Specifies whether |
| 1186 | .Xr sshd 8 |
| 1187 | should print |
| 1188 | .Pa /etc/motd |
| 1189 | when a user logs in interactively. |
| 1190 | (On some systems it is also printed by the shell, |
| 1191 | .Pa /etc/profile , |
| 1192 | or equivalent.) |
| 1193 | The default is |
| 1194 | .Dq yes . |
| 1195 | .It Cm Protocol |
| 1196 | Specifies the protocol versions |
| 1197 | .Xr sshd 8 |
| 1198 | supports. |
| 1199 | The possible values are |
| 1200 | .Sq 1 |
| 1201 | and |
| 1202 | .Sq 2 . |
| 1203 | Multiple versions must be comma-separated. |
| 1204 | The default is |
| 1205 | .Sq 2 . |
| 1206 | Note that the order of the protocol list does not indicate preference, |
| 1207 | because the client selects among multiple protocol versions offered |
| 1208 | by the server. |
| 1209 | Specifying |
| 1210 | .Dq 2,1 |
| 1211 | is identical to |
| 1212 | .Dq 1,2 . |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 1213 | .It Cm PubkeyAcceptedKeyTypes |
| 1214 | Specifies the key types that will be accepted for public key authentication |
| 1215 | as a comma-separated pattern list. |
| 1216 | The default |
| 1217 | .Dq * |
| 1218 | will allow all key types. |
| 1219 | The |
| 1220 | .Fl Q |
| 1221 | option of |
| 1222 | .Xr ssh 1 |
| 1223 | may be used to list supported key types. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1224 | .It Cm PubkeyAuthentication |
| 1225 | Specifies whether public key authentication is allowed. |
| 1226 | The default is |
| 1227 | .Dq yes . |
| 1228 | Note that this option applies to protocol version 2 only. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 1229 | .It Cm RekeyLimit |
| 1230 | Specifies the maximum amount of data that may be transmitted before the |
| 1231 | session key is renegotiated, optionally followed a maximum amount of |
| 1232 | time that may pass before the session key is renegotiated. |
| 1233 | The first argument is specified in bytes and may have a suffix of |
| 1234 | .Sq K , |
| 1235 | .Sq M , |
| 1236 | or |
| 1237 | .Sq G |
| 1238 | to indicate Kilobytes, Megabytes, or Gigabytes, respectively. |
| 1239 | The default is between |
| 1240 | .Sq 1G |
| 1241 | and |
| 1242 | .Sq 4G , |
| 1243 | depending on the cipher. |
| 1244 | The optional second value is specified in seconds and may use any of the |
| 1245 | units documented in the |
| 1246 | .Sx TIME FORMATS |
| 1247 | section. |
| 1248 | The default value for |
| 1249 | .Cm RekeyLimit |
| 1250 | is |
| 1251 | .Dq default none , |
| 1252 | which means that rekeying is performed after the cipher's default amount |
| 1253 | of data has been sent or received and no time based rekeying is done. |
| 1254 | This option applies to protocol version 2 only. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1255 | .It Cm RevokedKeys |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 1256 | Specifies revoked public keys. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1257 | Keys listed in this file will be refused for public key authentication. |
| 1258 | Note that if this file is not readable, then public key authentication will |
| 1259 | be refused for all users. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 1260 | Keys may be specified as a text file, listing one public key per line, or as |
| 1261 | an OpenSSH Key Revocation List (KRL) as generated by |
| 1262 | .Xr ssh-keygen 1 . |
| 1263 | For more information on KRLs, see the KEY REVOCATION LISTS section in |
| 1264 | .Xr ssh-keygen 1 . |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1265 | .It Cm RhostsRSAAuthentication |
| 1266 | Specifies whether rhosts or /etc/hosts.equiv authentication together |
| 1267 | with successful RSA host authentication is allowed. |
| 1268 | The default is |
| 1269 | .Dq no . |
| 1270 | This option applies to protocol version 1 only. |
| 1271 | .It Cm RSAAuthentication |
| 1272 | Specifies whether pure RSA authentication is allowed. |
| 1273 | The default is |
| 1274 | .Dq yes . |
| 1275 | This option applies to protocol version 1 only. |
| 1276 | .It Cm ServerKeyBits |
| 1277 | Defines the number of bits in the ephemeral protocol version 1 server key. |
| 1278 | The minimum value is 512, and the default is 1024. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 1279 | .It Cm StreamLocalBindMask |
| 1280 | Sets the octal file creation mode mask |
| 1281 | .Pq umask |
| 1282 | used when creating a Unix-domain socket file for local or remote |
| 1283 | port forwarding. |
| 1284 | This option is only used for port forwarding to a Unix-domain socket file. |
| 1285 | .Pp |
| 1286 | The default value is 0177, which creates a Unix-domain socket file that is |
| 1287 | readable and writable only by the owner. |
| 1288 | Note that not all operating systems honor the file mode on Unix-domain |
| 1289 | socket files. |
| 1290 | .It Cm StreamLocalBindUnlink |
| 1291 | Specifies whether to remove an existing Unix-domain socket file for local |
| 1292 | or remote port forwarding before creating a new one. |
| 1293 | If the socket file already exists and |
| 1294 | .Cm StreamLocalBindUnlink |
| 1295 | is not enabled, |
| 1296 | .Nm sshd |
| 1297 | will be unable to forward the port to the Unix-domain socket file. |
| 1298 | This option is only used for port forwarding to a Unix-domain socket file. |
| 1299 | .Pp |
| 1300 | The argument must be |
| 1301 | .Dq yes |
| 1302 | or |
| 1303 | .Dq no . |
| 1304 | The default is |
| 1305 | .Dq no . |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1306 | .It Cm StrictModes |
| 1307 | Specifies whether |
| 1308 | .Xr sshd 8 |
| 1309 | should check file modes and ownership of the |
| 1310 | user's files and home directory before accepting login. |
| 1311 | This is normally desirable because novices sometimes accidentally leave their |
| 1312 | directory or files world-writable. |
| 1313 | The default is |
| 1314 | .Dq yes . |
| 1315 | Note that this does not apply to |
| 1316 | .Cm ChrootDirectory , |
| 1317 | whose permissions and ownership are checked unconditionally. |
| 1318 | .It Cm Subsystem |
| 1319 | Configures an external subsystem (e.g. file transfer daemon). |
| 1320 | Arguments should be a subsystem name and a command (with optional arguments) |
| 1321 | to execute upon subsystem request. |
| 1322 | .Pp |
| 1323 | The command |
| 1324 | .Xr sftp-server 8 |
| 1325 | implements the |
| 1326 | .Dq sftp |
| 1327 | file transfer subsystem. |
| 1328 | .Pp |
| 1329 | Alternately the name |
| 1330 | .Dq internal-sftp |
| 1331 | implements an in-process |
| 1332 | .Dq sftp |
| 1333 | server. |
| 1334 | This may simplify configurations using |
| 1335 | .Cm ChrootDirectory |
| 1336 | to force a different filesystem root on clients. |
| 1337 | .Pp |
| 1338 | By default no subsystems are defined. |
| 1339 | Note that this option applies to protocol version 2 only. |
| 1340 | .It Cm SyslogFacility |
| 1341 | Gives the facility code that is used when logging messages from |
| 1342 | .Xr sshd 8 . |
| 1343 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, |
| 1344 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. |
| 1345 | The default is AUTH. |
| 1346 | .It Cm TCPKeepAlive |
| 1347 | Specifies whether the system should send TCP keepalive messages to the |
| 1348 | other side. |
| 1349 | If they are sent, death of the connection or crash of one |
| 1350 | of the machines will be properly noticed. |
| 1351 | However, this means that |
| 1352 | connections will die if the route is down temporarily, and some people |
| 1353 | find it annoying. |
| 1354 | On the other hand, if TCP keepalives are not sent, |
| 1355 | sessions may hang indefinitely on the server, leaving |
| 1356 | .Dq ghost |
| 1357 | users and consuming server resources. |
| 1358 | .Pp |
| 1359 | The default is |
| 1360 | .Dq yes |
| 1361 | (to send TCP keepalive messages), and the server will notice |
| 1362 | if the network goes down or the client host crashes. |
| 1363 | This avoids infinitely hanging sessions. |
| 1364 | .Pp |
| 1365 | To disable TCP keepalive messages, the value should be set to |
| 1366 | .Dq no . |
| 1367 | .It Cm TrustedUserCAKeys |
| 1368 | Specifies a file containing public keys of certificate authorities that are |
| 1369 | trusted to sign user certificates for authentication. |
| 1370 | Keys are listed one per line; empty lines and comments starting with |
| 1371 | .Ql # |
| 1372 | are allowed. |
| 1373 | If a certificate is presented for authentication and has its signing CA key |
| 1374 | listed in this file, then it may be used for authentication for any user |
| 1375 | listed in the certificate's principals list. |
| 1376 | Note that certificates that lack a list of principals will not be permitted |
| 1377 | for authentication using |
| 1378 | .Cm TrustedUserCAKeys . |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 1379 | For more details on certificates, see the CERTIFICATES section in |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1380 | .Xr ssh-keygen 1 . |
| 1381 | .It Cm UseDNS |
| 1382 | Specifies whether |
| 1383 | .Xr sshd 8 |
| 1384 | should look up the remote host name and check that |
| 1385 | the resolved host name for the remote IP address maps back to the |
| 1386 | very same IP address. |
| 1387 | The default is |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 1388 | .Dq no . |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1389 | .It Cm UseLogin |
| 1390 | Specifies whether |
| 1391 | .Xr login 1 |
| 1392 | is used for interactive login sessions. |
| 1393 | The default is |
| 1394 | .Dq no . |
| 1395 | Note that |
| 1396 | .Xr login 1 |
| 1397 | is never used for remote command execution. |
| 1398 | Note also, that if this is enabled, |
| 1399 | .Cm X11Forwarding |
| 1400 | will be disabled because |
| 1401 | .Xr login 1 |
| 1402 | does not know how to handle |
| 1403 | .Xr xauth 1 |
| 1404 | cookies. |
| 1405 | If |
| 1406 | .Cm UsePrivilegeSeparation |
| 1407 | is specified, it will be disabled after authentication. |
| 1408 | .It Cm UsePAM |
| 1409 | Enables the Pluggable Authentication Module interface. |
| 1410 | If set to |
| 1411 | .Dq yes |
| 1412 | this will enable PAM authentication using |
| 1413 | .Cm ChallengeResponseAuthentication |
| 1414 | and |
| 1415 | .Cm PasswordAuthentication |
| 1416 | in addition to PAM account and session module processing for all |
| 1417 | authentication types. |
| 1418 | .Pp |
| 1419 | Because PAM challenge-response authentication usually serves an equivalent |
| 1420 | role to password authentication, you should disable either |
| 1421 | .Cm PasswordAuthentication |
| 1422 | or |
| 1423 | .Cm ChallengeResponseAuthentication. |
| 1424 | .Pp |
| 1425 | If |
| 1426 | .Cm UsePAM |
| 1427 | is enabled, you will not be able to run |
| 1428 | .Xr sshd 8 |
| 1429 | as a non-root user. |
| 1430 | The default is |
| 1431 | .Dq no . |
| 1432 | .It Cm UsePrivilegeSeparation |
| 1433 | Specifies whether |
| 1434 | .Xr sshd 8 |
| 1435 | separates privileges by creating an unprivileged child process |
| 1436 | to deal with incoming network traffic. |
| 1437 | After successful authentication, another process will be created that has |
| 1438 | the privilege of the authenticated user. |
| 1439 | The goal of privilege separation is to prevent privilege |
| 1440 | escalation by containing any corruption within the unprivileged processes. |
| 1441 | The default is |
| 1442 | .Dq yes . |
| 1443 | If |
| 1444 | .Cm UsePrivilegeSeparation |
| 1445 | is set to |
| 1446 | .Dq sandbox |
| 1447 | then the pre-authentication unprivileged process is subject to additional |
| 1448 | restrictions. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 1449 | .It Cm VersionAddendum |
| 1450 | Optionally specifies additional text to append to the SSH protocol banner |
| 1451 | sent by the server upon connection. |
| 1452 | The default is |
| 1453 | .Dq none . |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 1454 | .It Cm X11DisplayOffset |
| 1455 | Specifies the first display number available for |
| 1456 | .Xr sshd 8 Ns 's |
| 1457 | X11 forwarding. |
| 1458 | This prevents sshd from interfering with real X11 servers. |
| 1459 | The default is 10. |
| 1460 | .It Cm X11Forwarding |
| 1461 | Specifies whether X11 forwarding is permitted. |
| 1462 | The argument must be |
| 1463 | .Dq yes |
| 1464 | or |
| 1465 | .Dq no . |
| 1466 | The default is |
| 1467 | .Dq no . |
| 1468 | .Pp |
| 1469 | When X11 forwarding is enabled, there may be additional exposure to |
| 1470 | the server and to client displays if the |
| 1471 | .Xr sshd 8 |
| 1472 | proxy display is configured to listen on the wildcard address (see |
| 1473 | .Cm X11UseLocalhost |
| 1474 | below), though this is not the default. |
| 1475 | Additionally, the authentication spoofing and authentication data |
| 1476 | verification and substitution occur on the client side. |
| 1477 | The security risk of using X11 forwarding is that the client's X11 |
| 1478 | display server may be exposed to attack when the SSH client requests |
| 1479 | forwarding (see the warnings for |
| 1480 | .Cm ForwardX11 |
| 1481 | in |
| 1482 | .Xr ssh_config 5 ) . |
| 1483 | A system administrator may have a stance in which they want to |
| 1484 | protect clients that may expose themselves to attack by unwittingly |
| 1485 | requesting X11 forwarding, which can warrant a |
| 1486 | .Dq no |
| 1487 | setting. |
| 1488 | .Pp |
| 1489 | Note that disabling X11 forwarding does not prevent users from |
| 1490 | forwarding X11 traffic, as users can always install their own forwarders. |
| 1491 | X11 forwarding is automatically disabled if |
| 1492 | .Cm UseLogin |
| 1493 | is enabled. |
| 1494 | .It Cm X11UseLocalhost |
| 1495 | Specifies whether |
| 1496 | .Xr sshd 8 |
| 1497 | should bind the X11 forwarding server to the loopback address or to |
| 1498 | the wildcard address. |
| 1499 | By default, |
| 1500 | sshd binds the forwarding server to the loopback address and sets the |
| 1501 | hostname part of the |
| 1502 | .Ev DISPLAY |
| 1503 | environment variable to |
| 1504 | .Dq localhost . |
| 1505 | This prevents remote hosts from connecting to the proxy display. |
| 1506 | However, some older X11 clients may not function with this |
| 1507 | configuration. |
| 1508 | .Cm X11UseLocalhost |
| 1509 | may be set to |
| 1510 | .Dq no |
| 1511 | to specify that the forwarding server should be bound to the wildcard |
| 1512 | address. |
| 1513 | The argument must be |
| 1514 | .Dq yes |
| 1515 | or |
| 1516 | .Dq no . |
| 1517 | The default is |
| 1518 | .Dq yes . |
| 1519 | .It Cm XAuthLocation |
| 1520 | Specifies the full pathname of the |
| 1521 | .Xr xauth 1 |
| 1522 | program. |
| 1523 | The default is |
| 1524 | .Pa /usr/X11R6/bin/xauth . |
| 1525 | .El |
| 1526 | .Sh TIME FORMATS |
| 1527 | .Xr sshd 8 |
| 1528 | command-line arguments and configuration file options that specify time |
| 1529 | may be expressed using a sequence of the form: |
| 1530 | .Sm off |
| 1531 | .Ar time Op Ar qualifier , |
| 1532 | .Sm on |
| 1533 | where |
| 1534 | .Ar time |
| 1535 | is a positive integer value and |
| 1536 | .Ar qualifier |
| 1537 | is one of the following: |
| 1538 | .Pp |
| 1539 | .Bl -tag -width Ds -compact -offset indent |
| 1540 | .It Aq Cm none |
| 1541 | seconds |
| 1542 | .It Cm s | Cm S |
| 1543 | seconds |
| 1544 | .It Cm m | Cm M |
| 1545 | minutes |
| 1546 | .It Cm h | Cm H |
| 1547 | hours |
| 1548 | .It Cm d | Cm D |
| 1549 | days |
| 1550 | .It Cm w | Cm W |
| 1551 | weeks |
| 1552 | .El |
| 1553 | .Pp |
| 1554 | Each member of the sequence is added together to calculate |
| 1555 | the total time value. |
| 1556 | .Pp |
| 1557 | Time format examples: |
| 1558 | .Pp |
| 1559 | .Bl -tag -width Ds -compact -offset indent |
| 1560 | .It 600 |
| 1561 | 600 seconds (10 minutes) |
| 1562 | .It 10m |
| 1563 | 10 minutes |
| 1564 | .It 1h30m |
| 1565 | 1 hour 30 minutes (90 minutes) |
| 1566 | .El |
| 1567 | .Sh FILES |
| 1568 | .Bl -tag -width Ds |
| 1569 | .It Pa /etc/ssh/sshd_config |
| 1570 | Contains configuration data for |
| 1571 | .Xr sshd 8 . |
| 1572 | This file should be writable by root only, but it is recommended |
| 1573 | (though not necessary) that it be world-readable. |
| 1574 | .El |
| 1575 | .Sh SEE ALSO |
| 1576 | .Xr sshd 8 |
| 1577 | .Sh AUTHORS |
| 1578 | OpenSSH is a derivative of the original and free |
| 1579 | ssh 1.2.12 release by Tatu Ylonen. |
| 1580 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, |
| 1581 | Theo de Raadt and Dug Song |
| 1582 | removed many bugs, re-added newer features and |
| 1583 | created OpenSSH. |
| 1584 | Markus Friedl contributed the support for SSH |
| 1585 | protocol versions 1.5 and 2.0. |
| 1586 | Niels Provos and Markus Friedl contributed support |
| 1587 | for privilege separation. |