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.8,v 1.278 2014/11/15 14:41:03 bentley Exp $ |
| 37 | .Dd $Mdocdate: November 15 2014 $ |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 38 | .Dt SSHD 8 |
| 39 | .Os |
| 40 | .Sh NAME |
| 41 | .Nm sshd |
| 42 | .Nd OpenSSH SSH daemon |
| 43 | .Sh SYNOPSIS |
| 44 | .Nm sshd |
| 45 | .Bk -words |
| 46 | .Op Fl 46DdeiqTt |
| 47 | .Op Fl b Ar bits |
| 48 | .Op Fl C Ar connection_spec |
| 49 | .Op Fl c Ar host_certificate_file |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 50 | .Op Fl E Ar log_file |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 51 | .Op Fl f Ar config_file |
| 52 | .Op Fl g Ar login_grace_time |
| 53 | .Op Fl h Ar host_key_file |
| 54 | .Op Fl k Ar key_gen_time |
| 55 | .Op Fl o Ar option |
| 56 | .Op Fl p Ar port |
| 57 | .Op Fl u Ar len |
| 58 | .Ek |
| 59 | .Sh DESCRIPTION |
| 60 | .Nm |
| 61 | (OpenSSH Daemon) is the daemon program for |
| 62 | .Xr ssh 1 . |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 63 | Together these programs replace rlogin and rsh, |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 64 | and provide secure encrypted communications between two untrusted hosts |
| 65 | over an insecure network. |
| 66 | .Pp |
| 67 | .Nm |
| 68 | listens for connections from clients. |
| 69 | It is normally started at boot from |
| 70 | .Pa /etc/rc . |
| 71 | It forks a new |
| 72 | daemon for each incoming connection. |
| 73 | The forked daemons handle |
| 74 | key exchange, encryption, authentication, command execution, |
| 75 | and data exchange. |
| 76 | .Pp |
| 77 | .Nm |
| 78 | can be configured using command-line options or a configuration file |
| 79 | (by default |
| 80 | .Xr sshd_config 5 ) ; |
| 81 | command-line options override values specified in the |
| 82 | configuration file. |
| 83 | .Nm |
| 84 | rereads its configuration file when it receives a hangup signal, |
| 85 | .Dv SIGHUP , |
| 86 | by executing itself with the name and options it was started with, e.g.\& |
| 87 | .Pa /usr/sbin/sshd . |
| 88 | .Pp |
| 89 | The options are as follows: |
| 90 | .Bl -tag -width Ds |
| 91 | .It Fl 4 |
| 92 | Forces |
| 93 | .Nm |
| 94 | to use IPv4 addresses only. |
| 95 | .It Fl 6 |
| 96 | Forces |
| 97 | .Nm |
| 98 | to use IPv6 addresses only. |
| 99 | .It Fl b Ar bits |
| 100 | Specifies the number of bits in the ephemeral protocol version 1 |
| 101 | server key (default 1024). |
| 102 | .It Fl C Ar connection_spec |
| 103 | Specify the connection parameters to use for the |
| 104 | .Fl T |
| 105 | extended test mode. |
| 106 | If provided, any |
| 107 | .Cm Match |
| 108 | directives in the configuration file |
| 109 | that would apply to the specified user, host, and address will be set before |
| 110 | the configuration is written to standard output. |
| 111 | The connection parameters are supplied as keyword=value pairs. |
| 112 | The keywords are |
| 113 | .Dq user , |
| 114 | .Dq host , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 115 | .Dq laddr , |
| 116 | .Dq lport , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 117 | and |
| 118 | .Dq addr . |
| 119 | All are required and may be supplied in any order, either with multiple |
| 120 | .Fl C |
| 121 | options or as a comma-separated list. |
| 122 | .It Fl c Ar host_certificate_file |
| 123 | Specifies a path to a certificate file to identify |
| 124 | .Nm |
| 125 | during key exchange. |
| 126 | The certificate file must match a host key file specified using the |
| 127 | .Fl h |
| 128 | option or the |
| 129 | .Cm HostKey |
| 130 | configuration directive. |
| 131 | .It Fl D |
| 132 | When this option is specified, |
| 133 | .Nm |
| 134 | will not detach and does not become a daemon. |
| 135 | This allows easy monitoring of |
| 136 | .Nm sshd . |
| 137 | .It Fl d |
| 138 | Debug mode. |
| 139 | The server sends verbose debug output to standard error, |
| 140 | and does not put itself in the background. |
| 141 | The server also will not fork and will only process one connection. |
| 142 | This option is only intended for debugging for the server. |
| 143 | Multiple |
| 144 | .Fl d |
| 145 | options increase the debugging level. |
| 146 | Maximum is 3. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 147 | .It Fl E Ar log_file |
| 148 | Append debug logs to |
| 149 | .Ar log_file |
| 150 | instead of the system log. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 151 | .It Fl e |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 152 | Write debug logs to standard error instead of the system log. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 153 | .It Fl f Ar config_file |
| 154 | Specifies the name of the configuration file. |
| 155 | The default is |
| 156 | .Pa /etc/ssh/sshd_config . |
| 157 | .Nm |
| 158 | refuses to start if there is no configuration file. |
| 159 | .It Fl g Ar login_grace_time |
| 160 | Gives the grace time for clients to authenticate themselves (default |
| 161 | 120 seconds). |
| 162 | If the client fails to authenticate the user within |
| 163 | this many seconds, the server disconnects and exits. |
| 164 | A value of zero indicates no limit. |
| 165 | .It Fl h Ar host_key_file |
| 166 | Specifies a file from which a host key is read. |
| 167 | This option must be given if |
| 168 | .Nm |
| 169 | is not run as root (as the normal |
| 170 | host key files are normally not readable by anyone but root). |
| 171 | The default is |
| 172 | .Pa /etc/ssh/ssh_host_key |
| 173 | for protocol version 1, and |
| 174 | .Pa /etc/ssh/ssh_host_dsa_key , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 175 | .Pa /etc/ssh/ssh_host_ecdsa_key . |
| 176 | .Pa /etc/ssh/ssh_host_ed25519_key |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 177 | and |
| 178 | .Pa /etc/ssh/ssh_host_rsa_key |
| 179 | for protocol version 2. |
| 180 | It is possible to have multiple host key files for |
| 181 | the different protocol versions and host key algorithms. |
| 182 | .It Fl i |
| 183 | Specifies that |
| 184 | .Nm |
| 185 | is being run from |
| 186 | .Xr inetd 8 . |
| 187 | .Nm |
| 188 | is normally not run |
| 189 | from inetd because it needs to generate the server key before it can |
| 190 | respond to the client, and this may take tens of seconds. |
| 191 | Clients would have to wait too long if the key was regenerated every time. |
| 192 | However, with small key sizes (e.g. 512) using |
| 193 | .Nm |
| 194 | from inetd may |
| 195 | be feasible. |
| 196 | .It Fl k Ar key_gen_time |
| 197 | Specifies how often the ephemeral protocol version 1 server key is |
| 198 | regenerated (default 3600 seconds, or one hour). |
| 199 | The motivation for regenerating the key fairly |
| 200 | often is that the key is not stored anywhere, and after about an hour |
| 201 | it becomes impossible to recover the key for decrypting intercepted |
| 202 | communications even if the machine is cracked into or physically |
| 203 | seized. |
| 204 | A value of zero indicates that the key will never be regenerated. |
| 205 | .It Fl o Ar option |
| 206 | Can be used to give options in the format used in the configuration file. |
| 207 | This is useful for specifying options for which there is no separate |
| 208 | command-line flag. |
| 209 | For full details of the options, and their values, see |
| 210 | .Xr sshd_config 5 . |
| 211 | .It Fl p Ar port |
| 212 | Specifies the port on which the server listens for connections |
| 213 | (default 22). |
| 214 | Multiple port options are permitted. |
| 215 | Ports specified in the configuration file with the |
| 216 | .Cm Port |
| 217 | option are ignored when a command-line port is specified. |
| 218 | Ports specified using the |
| 219 | .Cm ListenAddress |
| 220 | option override command-line ports. |
| 221 | .It Fl q |
| 222 | Quiet mode. |
| 223 | Nothing is sent to the system log. |
| 224 | Normally the beginning, |
| 225 | authentication, and termination of each connection is logged. |
| 226 | .It Fl T |
| 227 | Extended test mode. |
| 228 | Check the validity of the configuration file, output the effective configuration |
| 229 | to stdout and then exit. |
| 230 | Optionally, |
| 231 | .Cm Match |
| 232 | rules may be applied by specifying the connection parameters using one or more |
| 233 | .Fl C |
| 234 | options. |
| 235 | .It Fl t |
| 236 | Test mode. |
| 237 | Only check the validity of the configuration file and sanity of the keys. |
| 238 | This is useful for updating |
| 239 | .Nm |
| 240 | reliably as configuration options may change. |
| 241 | .It Fl u Ar len |
| 242 | This option is used to specify the size of the field |
| 243 | in the |
| 244 | .Li utmp |
| 245 | structure that holds the remote host name. |
| 246 | If the resolved host name is longer than |
| 247 | .Ar len , |
| 248 | the dotted decimal value will be used instead. |
| 249 | This allows hosts with very long host names that |
| 250 | overflow this field to still be uniquely identified. |
| 251 | Specifying |
| 252 | .Fl u0 |
| 253 | indicates that only dotted decimal addresses |
| 254 | should be put into the |
| 255 | .Pa utmp |
| 256 | file. |
| 257 | .Fl u0 |
| 258 | may also be used to prevent |
| 259 | .Nm |
| 260 | from making DNS requests unless the authentication |
| 261 | mechanism or configuration requires it. |
| 262 | Authentication mechanisms that may require DNS include |
| 263 | .Cm RhostsRSAAuthentication , |
| 264 | .Cm HostbasedAuthentication , |
| 265 | and using a |
| 266 | .Cm from="pattern-list" |
| 267 | option in a key file. |
| 268 | Configuration options that require DNS include using a |
| 269 | USER@HOST pattern in |
| 270 | .Cm AllowUsers |
| 271 | or |
| 272 | .Cm DenyUsers . |
| 273 | .El |
| 274 | .Sh AUTHENTICATION |
| 275 | The OpenSSH SSH daemon supports SSH protocols 1 and 2. |
| 276 | The default is to use protocol 2 only, |
| 277 | though this can be changed via the |
| 278 | .Cm Protocol |
| 279 | option in |
| 280 | .Xr sshd_config 5 . |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 281 | Protocol 2 supports DSA, ECDSA, Ed25519 and RSA keys; |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 282 | protocol 1 only supports RSA keys. |
| 283 | For both protocols, |
| 284 | each host has a host-specific key, |
| 285 | normally 2048 bits, |
| 286 | used to identify the host. |
| 287 | .Pp |
| 288 | Forward security for protocol 1 is provided through |
| 289 | an additional server key, |
| 290 | normally 768 bits, |
| 291 | generated when the server starts. |
| 292 | This key is normally regenerated every hour if it has been used, and |
| 293 | is never stored on disk. |
| 294 | Whenever a client connects, the daemon responds with its public |
| 295 | host and server keys. |
| 296 | The client compares the |
| 297 | RSA host key against its own database to verify that it has not changed. |
| 298 | The client then generates a 256-bit random number. |
| 299 | It encrypts this |
| 300 | random number using both the host key and the server key, and sends |
| 301 | the encrypted number to the server. |
| 302 | Both sides then use this |
| 303 | random number as a session key which is used to encrypt all further |
| 304 | communications in the session. |
| 305 | The rest of the session is encrypted |
| 306 | using a conventional cipher, currently Blowfish or 3DES, with 3DES |
| 307 | being used by default. |
| 308 | The client selects the encryption algorithm |
| 309 | to use from those offered by the server. |
| 310 | .Pp |
| 311 | For protocol 2, |
| 312 | forward security is provided through a Diffie-Hellman key agreement. |
| 313 | This key agreement results in a shared session key. |
| 314 | The rest of the session is encrypted using a symmetric cipher, currently |
| 315 | 128-bit AES, Blowfish, 3DES, CAST128, Arcfour, 192-bit AES, or 256-bit AES. |
| 316 | The client selects the encryption algorithm |
| 317 | to use from those offered by the server. |
| 318 | Additionally, session integrity is provided |
| 319 | through a cryptographic message authentication code |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 320 | (hmac-md5, hmac-sha1, umac-64, umac-128, hmac-ripemd160, |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 321 | hmac-sha2-256 or hmac-sha2-512). |
| 322 | .Pp |
| 323 | Finally, the server and the client enter an authentication dialog. |
| 324 | The client tries to authenticate itself using |
| 325 | host-based authentication, |
| 326 | public key authentication, |
| 327 | challenge-response authentication, |
| 328 | or password authentication. |
| 329 | .Pp |
| 330 | Regardless of the authentication type, the account is checked to |
| 331 | ensure that it is accessible. An account is not accessible if it is |
| 332 | locked, listed in |
| 333 | .Cm DenyUsers |
| 334 | or its group is listed in |
| 335 | .Cm DenyGroups |
| 336 | \&. The definition of a locked account is system dependant. Some platforms |
| 337 | have their own account database (eg AIX) and some modify the passwd field ( |
| 338 | .Ql \&*LK\&* |
| 339 | on Solaris and UnixWare, |
| 340 | .Ql \&* |
| 341 | on HP-UX, containing |
| 342 | .Ql Nologin |
| 343 | on Tru64, |
| 344 | a leading |
| 345 | .Ql \&*LOCKED\&* |
| 346 | on FreeBSD and a leading |
| 347 | .Ql \&! |
| 348 | on most Linuxes). |
| 349 | If there is a requirement to disable password authentication |
| 350 | for the account while allowing still public-key, then the passwd field |
| 351 | should be set to something other than these values (eg |
| 352 | .Ql NP |
| 353 | or |
| 354 | .Ql \&*NP\&* |
| 355 | ). |
| 356 | .Pp |
| 357 | If the client successfully authenticates itself, a dialog for |
| 358 | preparing the session is entered. |
| 359 | At this time the client may request |
| 360 | things like allocating a pseudo-tty, forwarding X11 connections, |
| 361 | forwarding TCP connections, or forwarding the authentication agent |
| 362 | connection over the secure channel. |
| 363 | .Pp |
| 364 | After this, the client either requests a shell or execution of a command. |
| 365 | The sides then enter session mode. |
| 366 | In this mode, either side may send |
| 367 | data at any time, and such data is forwarded to/from the shell or |
| 368 | command on the server side, and the user terminal in the client side. |
| 369 | .Pp |
| 370 | When the user program terminates and all forwarded X11 and other |
| 371 | connections have been closed, the server sends command exit status to |
| 372 | the client, and both sides exit. |
| 373 | .Sh LOGIN PROCESS |
| 374 | When a user successfully logs in, |
| 375 | .Nm |
| 376 | does the following: |
| 377 | .Bl -enum -offset indent |
| 378 | .It |
| 379 | If the login is on a tty, and no command has been specified, |
| 380 | prints last login time and |
| 381 | .Pa /etc/motd |
| 382 | (unless prevented in the configuration file or by |
| 383 | .Pa ~/.hushlogin ; |
| 384 | see the |
| 385 | .Sx FILES |
| 386 | section). |
| 387 | .It |
| 388 | If the login is on a tty, records login time. |
| 389 | .It |
| 390 | Checks |
| 391 | .Pa /etc/nologin ; |
| 392 | if it exists, prints contents and quits |
| 393 | (unless root). |
| 394 | .It |
| 395 | Changes to run with normal user privileges. |
| 396 | .It |
| 397 | Sets up basic environment. |
| 398 | .It |
| 399 | Reads the file |
| 400 | .Pa ~/.ssh/environment , |
| 401 | if it exists, and users are allowed to change their environment. |
| 402 | See the |
| 403 | .Cm PermitUserEnvironment |
| 404 | option in |
| 405 | .Xr sshd_config 5 . |
| 406 | .It |
| 407 | Changes to user's home directory. |
| 408 | .It |
| 409 | If |
| 410 | .Pa ~/.ssh/rc |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 411 | exists and the |
| 412 | .Xr sshd_config 5 |
| 413 | .Cm PermitUserRC |
| 414 | option is set, runs it; else if |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 415 | .Pa /etc/ssh/sshrc |
| 416 | exists, runs |
| 417 | it; otherwise runs xauth. |
| 418 | The |
| 419 | .Dq rc |
| 420 | files are given the X11 |
| 421 | authentication protocol and cookie in standard input. |
| 422 | See |
| 423 | .Sx SSHRC , |
| 424 | below. |
| 425 | .It |
| 426 | Runs user's shell or command. |
| 427 | .El |
| 428 | .Sh SSHRC |
| 429 | If the file |
| 430 | .Pa ~/.ssh/rc |
| 431 | exists, |
| 432 | .Xr sh 1 |
| 433 | runs it after reading the |
| 434 | environment files but before starting the user's shell or command. |
| 435 | It must not produce any output on stdout; stderr must be used |
| 436 | instead. |
| 437 | If X11 forwarding is in use, it will receive the "proto cookie" pair in |
| 438 | its standard input (and |
| 439 | .Ev DISPLAY |
| 440 | in its environment). |
| 441 | The script must call |
| 442 | .Xr xauth 1 |
| 443 | because |
| 444 | .Nm |
| 445 | will not run xauth automatically to add X11 cookies. |
| 446 | .Pp |
| 447 | The primary purpose of this file is to run any initialization routines |
| 448 | which may be needed before the user's home directory becomes |
| 449 | accessible; AFS is a particular example of such an environment. |
| 450 | .Pp |
| 451 | This file will probably contain some initialization code followed by |
| 452 | something similar to: |
| 453 | .Bd -literal -offset 3n |
| 454 | if read proto cookie && [ -n "$DISPLAY" ]; then |
| 455 | if [ `echo $DISPLAY | cut -c1-10` = 'localhost:' ]; then |
| 456 | # X11UseLocalhost=yes |
| 457 | echo add unix:`echo $DISPLAY | |
| 458 | cut -c11-` $proto $cookie |
| 459 | else |
| 460 | # X11UseLocalhost=no |
| 461 | echo add $DISPLAY $proto $cookie |
| 462 | fi | xauth -q - |
| 463 | fi |
| 464 | .Ed |
| 465 | .Pp |
| 466 | If this file does not exist, |
| 467 | .Pa /etc/ssh/sshrc |
| 468 | is run, and if that |
| 469 | does not exist either, xauth is used to add the cookie. |
| 470 | .Sh AUTHORIZED_KEYS FILE FORMAT |
| 471 | .Cm AuthorizedKeysFile |
| 472 | specifies the files containing public keys for |
| 473 | public key authentication; |
| 474 | if none is specified, the default is |
| 475 | .Pa ~/.ssh/authorized_keys |
| 476 | and |
| 477 | .Pa ~/.ssh/authorized_keys2 . |
| 478 | Each line of the file contains one |
| 479 | key (empty lines and lines starting with a |
| 480 | .Ql # |
| 481 | are ignored as |
| 482 | comments). |
| 483 | Protocol 1 public keys consist of the following space-separated fields: |
| 484 | options, bits, exponent, modulus, comment. |
| 485 | Protocol 2 public key consist of: |
| 486 | options, keytype, base64-encoded key, comment. |
| 487 | The options field is optional; |
| 488 | its presence is determined by whether the line starts |
| 489 | with a number or not (the options field never starts with a number). |
| 490 | The bits, exponent, modulus, and comment fields give the RSA key for |
| 491 | protocol version 1; the |
| 492 | comment field is not used for anything (but may be convenient for the |
| 493 | user to identify the key). |
| 494 | For protocol version 2 the keytype is |
| 495 | .Dq ecdsa-sha2-nistp256 , |
| 496 | .Dq ecdsa-sha2-nistp384 , |
| 497 | .Dq ecdsa-sha2-nistp521 , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 498 | .Dq ssh-ed25519 , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 499 | .Dq ssh-dss |
| 500 | or |
| 501 | .Dq ssh-rsa . |
| 502 | .Pp |
| 503 | Note that lines in this file are usually several hundred bytes long |
| 504 | (because of the size of the public key encoding) up to a limit of |
| 505 | 8 kilobytes, which permits DSA keys up to 8 kilobits and RSA |
| 506 | keys up to 16 kilobits. |
| 507 | You don't want to type them in; instead, copy the |
| 508 | .Pa identity.pub , |
| 509 | .Pa id_dsa.pub , |
| 510 | .Pa id_ecdsa.pub , |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 511 | .Pa id_ed25519.pub , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 512 | or the |
| 513 | .Pa id_rsa.pub |
| 514 | file and edit it. |
| 515 | .Pp |
| 516 | .Nm |
| 517 | enforces a minimum RSA key modulus size for protocol 1 |
| 518 | and protocol 2 keys of 768 bits. |
| 519 | .Pp |
| 520 | The options (if present) consist of comma-separated option |
| 521 | specifications. |
| 522 | No spaces are permitted, except within double quotes. |
| 523 | The following option specifications are supported (note |
| 524 | that option keywords are case-insensitive): |
| 525 | .Bl -tag -width Ds |
| 526 | .It Cm cert-authority |
| 527 | Specifies that the listed key is a certification authority (CA) that is |
| 528 | trusted to validate signed certificates for user authentication. |
| 529 | .Pp |
| 530 | Certificates may encode access restrictions similar to these key options. |
| 531 | If both certificate restrictions and key options are present, the most |
| 532 | restrictive union of the two is applied. |
| 533 | .It Cm command="command" |
| 534 | Specifies that the command is executed whenever this key is used for |
| 535 | authentication. |
| 536 | The command supplied by the user (if any) is ignored. |
| 537 | The command is run on a pty if the client requests a pty; |
| 538 | otherwise it is run without a tty. |
| 539 | If an 8-bit clean channel is required, |
| 540 | one must not request a pty or should specify |
| 541 | .Cm no-pty . |
| 542 | A quote may be included in the command by quoting it with a backslash. |
| 543 | This option might be useful |
| 544 | to restrict certain public keys to perform just a specific operation. |
| 545 | An example might be a key that permits remote backups but nothing else. |
| 546 | Note that the client may specify TCP and/or X11 |
| 547 | forwarding unless they are explicitly prohibited. |
| 548 | The command originally supplied by the client is available in the |
| 549 | .Ev SSH_ORIGINAL_COMMAND |
| 550 | environment variable. |
| 551 | Note that this option applies to shell, command or subsystem execution. |
| 552 | Also note that this command may be superseded by either a |
| 553 | .Xr sshd_config 5 |
| 554 | .Cm ForceCommand |
| 555 | directive or a command embedded in a certificate. |
| 556 | .It Cm environment="NAME=value" |
| 557 | Specifies that the string is to be added to the environment when |
| 558 | logging in using this key. |
| 559 | Environment variables set this way |
| 560 | override other default environment values. |
| 561 | Multiple options of this type are permitted. |
| 562 | Environment processing is disabled by default and is |
| 563 | controlled via the |
| 564 | .Cm PermitUserEnvironment |
| 565 | option. |
| 566 | This option is automatically disabled if |
| 567 | .Cm UseLogin |
| 568 | is enabled. |
| 569 | .It Cm from="pattern-list" |
| 570 | Specifies that in addition to public key authentication, either the canonical |
| 571 | name of the remote host or its IP address must be present in the |
| 572 | comma-separated list of patterns. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 573 | See PATTERNS in |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 574 | .Xr ssh_config 5 |
| 575 | for more information on patterns. |
| 576 | .Pp |
| 577 | In addition to the wildcard matching that may be applied to hostnames or |
| 578 | addresses, a |
| 579 | .Cm from |
| 580 | stanza may match IP addresses using CIDR address/masklen notation. |
| 581 | .Pp |
| 582 | The purpose of this option is to optionally increase security: public key |
| 583 | authentication by itself does not trust the network or name servers or |
| 584 | anything (but the key); however, if somebody somehow steals the key, the key |
| 585 | permits an intruder to log in from anywhere in the world. |
| 586 | This additional option makes using a stolen key more difficult (name |
| 587 | servers and/or routers would have to be compromised in addition to |
| 588 | just the key). |
| 589 | .It Cm no-agent-forwarding |
| 590 | Forbids authentication agent forwarding when this key is used for |
| 591 | authentication. |
| 592 | .It Cm no-port-forwarding |
| 593 | Forbids TCP forwarding when this key is used for authentication. |
| 594 | Any port forward requests by the client will return an error. |
| 595 | This might be used, e.g. in connection with the |
| 596 | .Cm command |
| 597 | option. |
| 598 | .It Cm no-pty |
| 599 | Prevents tty allocation (a request to allocate a pty will fail). |
| 600 | .It Cm no-user-rc |
| 601 | Disables execution of |
| 602 | .Pa ~/.ssh/rc . |
| 603 | .It Cm no-X11-forwarding |
| 604 | Forbids X11 forwarding when this key is used for authentication. |
| 605 | Any X11 forward requests by the client will return an error. |
| 606 | .It Cm permitopen="host:port" |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 607 | Limit local port forwarding with |
| 608 | .Xr ssh 1 |
| 609 | .Fl L |
| 610 | such that it may only connect to the specified host and port. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 611 | IPv6 addresses can be specified by enclosing the address in square brackets. |
| 612 | Multiple |
| 613 | .Cm permitopen |
| 614 | options may be applied separated by commas. |
| 615 | No pattern matching is performed on the specified hostnames, |
| 616 | they must be literal domains or addresses. |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 617 | A port specification of |
| 618 | .Cm * |
| 619 | matches any port. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 620 | .It Cm principals="principals" |
| 621 | On a |
| 622 | .Cm cert-authority |
| 623 | line, specifies allowed principals for certificate authentication as a |
| 624 | comma-separated list. |
| 625 | At least one name from the list must appear in the certificate's |
| 626 | list of principals for the certificate to be accepted. |
| 627 | This option is ignored for keys that are not marked as trusted certificate |
| 628 | signers using the |
| 629 | .Cm cert-authority |
| 630 | option. |
| 631 | .It Cm tunnel="n" |
| 632 | Force a |
| 633 | .Xr tun 4 |
| 634 | device on the server. |
| 635 | Without this option, the next available device will be used if |
| 636 | the client requests a tunnel. |
| 637 | .El |
| 638 | .Pp |
| 639 | An example authorized_keys file: |
| 640 | .Bd -literal -offset 3n |
| 641 | # Comments allowed at start of line |
| 642 | ssh-rsa AAAAB3Nza...LiPk== user@example.net |
| 643 | from="*.sales.example.net,!pc.sales.example.net" ssh-rsa |
| 644 | AAAAB2...19Q== john@example.net |
| 645 | command="dump /home",no-pty,no-port-forwarding ssh-dss |
| 646 | AAAAC3...51R== example.net |
| 647 | permitopen="192.0.2.1:80",permitopen="192.0.2.2:25" ssh-dss |
| 648 | AAAAB5...21S== |
| 649 | tunnel="0",command="sh /etc/netstart tun0" ssh-rsa AAAA...== |
| 650 | jane@example.net |
| 651 | .Ed |
| 652 | .Sh SSH_KNOWN_HOSTS FILE FORMAT |
| 653 | The |
| 654 | .Pa /etc/ssh/ssh_known_hosts |
| 655 | and |
| 656 | .Pa ~/.ssh/known_hosts |
| 657 | files contain host public keys for all known hosts. |
| 658 | The global file should |
| 659 | be prepared by the administrator (optional), and the per-user file is |
| 660 | maintained automatically: whenever the user connects from an unknown host, |
| 661 | its key is added to the per-user file. |
| 662 | .Pp |
| 663 | Each line in these files contains the following fields: markers (optional), |
| 664 | hostnames, bits, exponent, modulus, comment. |
| 665 | The fields are separated by spaces. |
| 666 | .Pp |
| 667 | The marker is optional, but if it is present then it must be one of |
| 668 | .Dq @cert-authority , |
| 669 | to indicate that the line contains a certification authority (CA) key, |
| 670 | or |
| 671 | .Dq @revoked , |
| 672 | to indicate that the key contained on the line is revoked and must not ever |
| 673 | be accepted. |
| 674 | Only one marker should be used on a key line. |
| 675 | .Pp |
| 676 | Hostnames is a comma-separated list of patterns |
| 677 | .Pf ( Ql * |
| 678 | and |
| 679 | .Ql \&? |
| 680 | act as |
| 681 | wildcards); each pattern in turn is matched against the canonical host |
| 682 | name (when authenticating a client) or against the user-supplied |
| 683 | name (when authenticating a server). |
| 684 | A pattern may also be preceded by |
| 685 | .Ql \&! |
| 686 | to indicate negation: if the host name matches a negated |
| 687 | pattern, it is not accepted (by that line) even if it matched another |
| 688 | pattern on the line. |
| 689 | A hostname or address may optionally be enclosed within |
| 690 | .Ql \&[ |
| 691 | and |
| 692 | .Ql \&] |
| 693 | brackets then followed by |
| 694 | .Ql \&: |
| 695 | and a non-standard port number. |
| 696 | .Pp |
| 697 | Alternately, hostnames may be stored in a hashed form which hides host names |
| 698 | and addresses should the file's contents be disclosed. |
| 699 | Hashed hostnames start with a |
| 700 | .Ql | |
| 701 | character. |
| 702 | Only one hashed hostname may appear on a single line and none of the above |
| 703 | negation or wildcard operators may be applied. |
| 704 | .Pp |
| 705 | Bits, exponent, and modulus are taken directly from the RSA host key; they |
| 706 | can be obtained, for example, from |
| 707 | .Pa /etc/ssh/ssh_host_key.pub . |
| 708 | The optional comment field continues to the end of the line, and is not used. |
| 709 | .Pp |
| 710 | Lines starting with |
| 711 | .Ql # |
| 712 | and empty lines are ignored as comments. |
| 713 | .Pp |
| 714 | When performing host authentication, authentication is accepted if any |
| 715 | matching line has the proper key; either one that matches exactly or, |
| 716 | if the server has presented a certificate for authentication, the key |
| 717 | of the certification authority that signed the certificate. |
| 718 | For a key to be trusted as a certification authority, it must use the |
| 719 | .Dq @cert-authority |
| 720 | marker described above. |
| 721 | .Pp |
| 722 | The known hosts file also provides a facility to mark keys as revoked, |
| 723 | for example when it is known that the associated private key has been |
| 724 | stolen. |
| 725 | Revoked keys are specified by including the |
| 726 | .Dq @revoked |
| 727 | marker at the beginning of the key line, and are never accepted for |
| 728 | authentication or as certification authorities, but instead will |
| 729 | produce a warning from |
| 730 | .Xr ssh 1 |
| 731 | when they are encountered. |
| 732 | .Pp |
| 733 | It is permissible (but not |
| 734 | recommended) to have several lines or different host keys for the same |
| 735 | names. |
| 736 | This will inevitably happen when short forms of host names |
| 737 | from different domains are put in the file. |
| 738 | It is possible |
| 739 | that the files contain conflicting information; authentication is |
| 740 | accepted if valid information can be found from either file. |
| 741 | .Pp |
| 742 | Note that the lines in these files are typically hundreds of characters |
| 743 | long, and you definitely don't want to type in the host keys by hand. |
| 744 | Rather, generate them by a script, |
| 745 | .Xr ssh-keyscan 1 |
| 746 | or by taking |
| 747 | .Pa /etc/ssh/ssh_host_key.pub |
| 748 | and adding the host names at the front. |
| 749 | .Xr ssh-keygen 1 |
| 750 | also offers some basic automated editing for |
| 751 | .Pa ~/.ssh/known_hosts |
| 752 | including removing hosts matching a host name and converting all host |
| 753 | names to their hashed representations. |
| 754 | .Pp |
| 755 | An example ssh_known_hosts file: |
| 756 | .Bd -literal -offset 3n |
| 757 | # Comments allowed at start of line |
| 758 | closenet,...,192.0.2.53 1024 37 159...93 closenet.example.net |
| 759 | cvs.example.net,192.0.2.10 ssh-rsa AAAA1234.....= |
| 760 | # A hashed hostname |
| 761 | |1|JfKTdBh7rNbXkVAQCRp4OQoPfmI=|USECr3SWf1JUPsms5AqfD5QfxkM= ssh-rsa |
| 762 | AAAA1234.....= |
| 763 | # A revoked key |
| 764 | @revoked * ssh-rsa AAAAB5W... |
| 765 | # A CA key, accepted for any host in *.mydomain.com or *.mydomain.org |
| 766 | @cert-authority *.mydomain.org,*.mydomain.com ssh-rsa AAAAB5W... |
| 767 | .Ed |
| 768 | .Sh FILES |
| 769 | .Bl -tag -width Ds -compact |
| 770 | .It Pa ~/.hushlogin |
| 771 | This file is used to suppress printing the last login time and |
| 772 | .Pa /etc/motd , |
| 773 | if |
| 774 | .Cm PrintLastLog |
| 775 | and |
| 776 | .Cm PrintMotd , |
| 777 | respectively, |
| 778 | are enabled. |
| 779 | It does not suppress printing of the banner specified by |
| 780 | .Cm Banner . |
| 781 | .Pp |
| 782 | .It Pa ~/.rhosts |
| 783 | This file is used for host-based authentication (see |
| 784 | .Xr ssh 1 |
| 785 | for more information). |
| 786 | On some machines this file may need to be |
| 787 | world-readable if the user's home directory is on an NFS partition, |
| 788 | because |
| 789 | .Nm |
| 790 | reads it as root. |
| 791 | Additionally, this file must be owned by the user, |
| 792 | and must not have write permissions for anyone else. |
| 793 | The recommended |
| 794 | permission for most machines is read/write for the user, and not |
| 795 | accessible by others. |
| 796 | .Pp |
| 797 | .It Pa ~/.shosts |
| 798 | This file is used in exactly the same way as |
| 799 | .Pa .rhosts , |
| 800 | but allows host-based authentication without permitting login with |
| 801 | rlogin/rsh. |
| 802 | .Pp |
| 803 | .It Pa ~/.ssh/ |
| 804 | This directory is the default location for all user-specific configuration |
| 805 | and authentication information. |
| 806 | There is no general requirement to keep the entire contents of this directory |
| 807 | secret, but the recommended permissions are read/write/execute for the user, |
| 808 | and not accessible by others. |
| 809 | .Pp |
| 810 | .It Pa ~/.ssh/authorized_keys |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 811 | Lists the public keys (DSA, ECDSA, Ed25519, RSA) |
| 812 | that can be used for logging in as this user. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 813 | The format of this file is described above. |
| 814 | The content of the file is not highly sensitive, but the recommended |
| 815 | permissions are read/write for the user, and not accessible by others. |
| 816 | .Pp |
| 817 | If this file, the |
| 818 | .Pa ~/.ssh |
| 819 | directory, or the user's home directory are writable |
| 820 | by other users, then the file could be modified or replaced by unauthorized |
| 821 | users. |
| 822 | In this case, |
| 823 | .Nm |
| 824 | will not allow it to be used unless the |
| 825 | .Cm StrictModes |
| 826 | option has been set to |
| 827 | .Dq no . |
| 828 | .Pp |
| 829 | .It Pa ~/.ssh/environment |
| 830 | This file is read into the environment at login (if it exists). |
| 831 | It can only contain empty lines, comment lines (that start with |
| 832 | .Ql # ) , |
| 833 | and assignment lines of the form name=value. |
| 834 | The file should be writable |
| 835 | only by the user; it need not be readable by anyone else. |
| 836 | Environment processing is disabled by default and is |
| 837 | controlled via the |
| 838 | .Cm PermitUserEnvironment |
| 839 | option. |
| 840 | .Pp |
| 841 | .It Pa ~/.ssh/known_hosts |
| 842 | Contains a list of host keys for all hosts the user has logged into |
| 843 | that are not already in the systemwide list of known host keys. |
| 844 | The format of this file is described above. |
| 845 | This file should be writable only by root/the owner and |
| 846 | can, but need not be, world-readable. |
| 847 | .Pp |
| 848 | .It Pa ~/.ssh/rc |
| 849 | Contains initialization routines to be run before |
| 850 | the user's home directory becomes accessible. |
| 851 | This file should be writable only by the user, and need not be |
| 852 | readable by anyone else. |
| 853 | .Pp |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 854 | .It Pa /etc/hosts.equiv |
| 855 | This file is for host-based authentication (see |
| 856 | .Xr ssh 1 ) . |
| 857 | It should only be writable by root. |
| 858 | .Pp |
| 859 | .It Pa /etc/moduli |
| 860 | Contains Diffie-Hellman groups used for the "Diffie-Hellman Group Exchange". |
| 861 | The file format is described in |
| 862 | .Xr moduli 5 . |
| 863 | .Pp |
| 864 | .It Pa /etc/motd |
| 865 | See |
| 866 | .Xr motd 5 . |
| 867 | .Pp |
| 868 | .It Pa /etc/nologin |
| 869 | If this file exists, |
| 870 | .Nm |
| 871 | refuses to let anyone except root log in. |
| 872 | The contents of the file |
| 873 | are displayed to anyone trying to log in, and non-root connections are |
| 874 | refused. |
| 875 | The file should be world-readable. |
| 876 | .Pp |
| 877 | .It Pa /etc/shosts.equiv |
| 878 | This file is used in exactly the same way as |
| 879 | .Pa hosts.equiv , |
| 880 | but allows host-based authentication without permitting login with |
| 881 | rlogin/rsh. |
| 882 | .Pp |
| 883 | .It Pa /etc/ssh/ssh_host_key |
| 884 | .It Pa /etc/ssh/ssh_host_dsa_key |
| 885 | .It Pa /etc/ssh/ssh_host_ecdsa_key |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 886 | .It Pa /etc/ssh/ssh_host_ed25519_key |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 887 | .It Pa /etc/ssh/ssh_host_rsa_key |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 888 | These files contain the private parts of the host keys. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 889 | These files should only be owned by root, readable only by root, and not |
| 890 | accessible to others. |
| 891 | Note that |
| 892 | .Nm |
| 893 | does not start if these files are group/world-accessible. |
| 894 | .Pp |
| 895 | .It Pa /etc/ssh/ssh_host_key.pub |
| 896 | .It Pa /etc/ssh/ssh_host_dsa_key.pub |
| 897 | .It Pa /etc/ssh/ssh_host_ecdsa_key.pub |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 898 | .It Pa /etc/ssh/ssh_host_ed25519_key.pub |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 899 | .It Pa /etc/ssh/ssh_host_rsa_key.pub |
Adam Langley | d059297 | 2015-03-30 14:49:51 -0700 | [diff] [blame] | 900 | These files contain the public parts of the host keys. |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 901 | These files should be world-readable but writable only by |
| 902 | root. |
| 903 | Their contents should match the respective private parts. |
| 904 | These files are not |
| 905 | really used for anything; they are provided for the convenience of |
| 906 | the user so their contents can be copied to known hosts files. |
| 907 | These files are created using |
| 908 | .Xr ssh-keygen 1 . |
| 909 | .Pp |
| 910 | .It Pa /etc/ssh/ssh_known_hosts |
| 911 | Systemwide list of known host keys. |
| 912 | This file should be prepared by the |
| 913 | system administrator to contain the public host keys of all machines in the |
| 914 | organization. |
| 915 | The format of this file is described above. |
| 916 | This file should be writable only by root/the owner and |
| 917 | should be world-readable. |
| 918 | .Pp |
| 919 | .It Pa /etc/ssh/sshd_config |
| 920 | Contains configuration data for |
| 921 | .Nm sshd . |
| 922 | The file format and configuration options are described in |
| 923 | .Xr sshd_config 5 . |
| 924 | .Pp |
| 925 | .It Pa /etc/ssh/sshrc |
| 926 | Similar to |
| 927 | .Pa ~/.ssh/rc , |
| 928 | it can be used to specify |
| 929 | machine-specific login-time initializations globally. |
| 930 | This file should be writable only by root, and should be world-readable. |
| 931 | .Pp |
| 932 | .It Pa /var/empty |
| 933 | .Xr chroot 2 |
| 934 | directory used by |
| 935 | .Nm |
| 936 | during privilege separation in the pre-authentication phase. |
| 937 | The directory should not contain any files and must be owned by root |
| 938 | and not group or world-writable. |
| 939 | .Pp |
| 940 | .It Pa /var/run/sshd.pid |
| 941 | Contains the process ID of the |
| 942 | .Nm |
| 943 | listening for connections (if there are several daemons running |
| 944 | concurrently for different ports, this contains the process ID of the one |
| 945 | started last). |
| 946 | The content of this file is not sensitive; it can be world-readable. |
| 947 | .El |
| 948 | .Sh SEE ALSO |
| 949 | .Xr scp 1 , |
| 950 | .Xr sftp 1 , |
| 951 | .Xr ssh 1 , |
| 952 | .Xr ssh-add 1 , |
| 953 | .Xr ssh-agent 1 , |
| 954 | .Xr ssh-keygen 1 , |
| 955 | .Xr ssh-keyscan 1 , |
| 956 | .Xr chroot 2 , |
Greg Hartman | bd77cf7 | 2015-02-25 13:21:06 -0800 | [diff] [blame] | 957 | .Xr login.conf 5 , |
| 958 | .Xr moduli 5 , |
| 959 | .Xr sshd_config 5 , |
| 960 | .Xr inetd 8 , |
| 961 | .Xr sftp-server 8 |
| 962 | .Sh AUTHORS |
| 963 | OpenSSH is a derivative of the original and free |
| 964 | ssh 1.2.12 release by Tatu Ylonen. |
| 965 | Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, |
| 966 | Theo de Raadt and Dug Song |
| 967 | removed many bugs, re-added newer features and |
| 968 | created OpenSSH. |
| 969 | Markus Friedl contributed the support for SSH |
| 970 | protocol versions 1.5 and 2.0. |
| 971 | Niels Provos and Markus Friedl contributed support |
| 972 | for privilege separation. |