Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 1 | .\" -*- nroff -*- |
| 2 | .\" |
| 3 | .\" sshd.8.in |
| 4 | .\" |
| 5 | .\" Author: Tatu Ylonen <ylo@cs.hut.fi> |
| 6 | .\" |
| 7 | .\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland |
| 8 | .\" All rights reserved |
| 9 | .\" |
| 10 | .\" Created: Sat Apr 22 21:55:14 1995 ylo |
| 11 | .\" |
Damien Miller | 3226509 | 1999-11-12 11:33:04 +1100 | [diff] [blame] | 12 | .\" $Id: sshd.8,v 1.7 1999/11/12 00:33:04 damien Exp $ |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 13 | .\" |
| 14 | .Dd September 25, 1999 |
| 15 | .Dt SSHD 8 |
| 16 | .Os |
| 17 | .Sh NAME |
| 18 | .Nm sshd |
| 19 | .Nd secure shell daemon |
| 20 | .Sh SYNOPSIS |
| 21 | .Nm sshd |
| 22 | .Op Fl diq |
| 23 | .Op Fl b Ar bits |
| 24 | .Op Fl f Ar config_file |
| 25 | .Op Fl g Ar login_grace_time |
| 26 | .Op Fl h Ar host_key_file |
| 27 | .Op Fl k Ar key_gen_time |
| 28 | .Op Fl p Ar port |
| 29 | .Sh DESCRIPTION |
| 30 | .Nm |
| 31 | (Secure Shell Daemon) is the daemon program for |
| 32 | .Xr ssh 1 . |
| 33 | Together these programs replace rlogin and rsh programs, and |
| 34 | provide secure encrypted communications between two untrusted hosts |
| 35 | over an insecure network. The programs are intended to be as easy to |
| 36 | install and use as possible. |
| 37 | .Pp |
| 38 | .Nm |
| 39 | is the daemon that listens for connections from clients. It is |
| 40 | normally started at boot from |
| 41 | .Pa /etc/rc . |
| 42 | It forks a new |
| 43 | daemon for each incoming connection. The forked daemons handle |
| 44 | key exchange, encryption, authentication, command execution, |
| 45 | and data exchange. |
| 46 | .Pp |
| 47 | .Nm |
| 48 | works as follows. Each host has a host-specific RSA key |
| 49 | (normally 1024 bits) used to identify the host. Additionally, when |
| 50 | the daemon starts, it generates a server RSA key (normally 768 bits). |
| 51 | This key is normally regenerated every hour if it has been used, and |
| 52 | is never stored on disk. |
| 53 | .Pp |
| 54 | Whenever a client connects the daemon, the daemon sends its host |
| 55 | and server public keys to the client. The client compares the |
| 56 | host key against its own database to verify that it has not changed. |
| 57 | The client then generates a 256 bit random number. It encrypts this |
| 58 | random number using both the host key and the server key, and sends |
| 59 | the encrypted number to the server. Both sides then start to use this |
| 60 | random number as a session key which is used to encrypt all further |
| 61 | communications in the session. The rest of the session is encrypted |
| 62 | using a conventional cipher, currently Blowfish and 3DES, with 3DES |
| 63 | being is used by default. The client selects the encryption algorithm |
| 64 | to use from those offered by the server. |
| 65 | .Pp |
| 66 | Next, the server and the client enter an authentication dialog. The |
| 67 | client tries to authenticate itself using |
| 68 | .Pa .rhosts |
| 69 | authentication, |
| 70 | .Pa .rhosts |
| 71 | authentication combined with RSA host |
| 72 | authentication, RSA challenge-response authentication, or password |
| 73 | based authentication. |
| 74 | .Pp |
| 75 | Rhosts authentication is normally disabled |
| 76 | because it is fundamentally insecure, but can be enabled in the server |
| 77 | configuration file if desired. System security is not improved unless |
| 78 | .Xr rshd 8 , |
| 79 | .Xr rlogind 8 , |
| 80 | .Xr rexecd 8 , |
| 81 | and |
| 82 | .Xr rexd 8 |
| 83 | are disabled (thus completely disabling |
| 84 | .Xr rlogin 1 |
| 85 | and |
| 86 | .Xr rsh 1 |
| 87 | into that machine). |
| 88 | .Pp |
| 89 | If the client successfully authenticates itself, a dialog for |
| 90 | preparing the session is entered. At this time the client may request |
| 91 | things like allocating a pseudo-tty, forwarding X11 connections, |
| 92 | forwarding TCP/IP connections, or forwarding the authentication agent |
| 93 | connection over the secure channel. |
| 94 | .Pp |
| 95 | Finally, the client either requests a shell or execution of a command. |
| 96 | The sides then enter session mode. In this mode, either side may send |
| 97 | data at any time, and such data is forwarded to/from the shell or |
| 98 | command on the server side, and the user terminal in the client side. |
| 99 | .Pp |
| 100 | When the user program terminates and all forwarded X11 and other |
| 101 | connections have been closed, the server sends command exit status to |
| 102 | the client, and both sides exit. |
| 103 | .Pp |
| 104 | .Nm |
| 105 | can be configured using command-line options or a configuration |
| 106 | file. Command-line options override values specified in the |
| 107 | configuration file. |
| 108 | .Pp |
| 109 | The options are as follows: |
| 110 | .Bl -tag -width Ds |
| 111 | .It Fl b Ar bits |
| 112 | Specifies the number of bits in the server key (default 768). |
| 113 | .Pp |
| 114 | .It Fl d |
| 115 | Debug mode. The server sends verbose debug output to the system |
| 116 | log, and does not put itself in the background. The server also will |
| 117 | not fork and will only process one connection. This option is only |
| 118 | intended for debugging for the server. |
| 119 | .It Fl f Ar configuration_file |
| 120 | Specifies the name of the configuration file. The default is |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 121 | .Pa /etc/ssh/sshd_config . |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 122 | .Nm |
| 123 | refuses to start if there is no configuration file. |
| 124 | .It Fl g Ar login_grace_time |
| 125 | Gives the grace time for clients to authenticate themselves (default |
| 126 | 300 seconds). If the client fails to authenticate the user within |
| 127 | this many seconds, the server disconnects and exits. A value of zero |
| 128 | indicates no limit. |
| 129 | .It Fl h Ar host_key_file |
| 130 | Specifies the file from which the host key is read (default |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 131 | .Pa /etc/ssh/ssh_host_key ) . |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 132 | This option must be given if |
| 133 | .Nm |
| 134 | is not run as root (as the normal |
| 135 | host file is normally not readable by anyone but root). |
| 136 | .It Fl i |
| 137 | Specifies that |
| 138 | .Nm |
| 139 | is being run from inetd. |
| 140 | .Nm |
| 141 | is normally not run |
| 142 | from inetd because it needs to generate the server key before it can |
| 143 | respond to the client, and this may take tens of seconds. Clients |
| 144 | would have to wait too long if the key was regenerated every time. |
| 145 | However, with small key sizes (e.g. 512) using |
| 146 | .Nm |
| 147 | from inetd may |
| 148 | be feasible. |
| 149 | .It Fl k Ar key_gen_time |
| 150 | Specifies how often the server key is regenerated (default 3600 |
| 151 | seconds, or one hour). The motivation for regenerating the key fairly |
| 152 | often is that the key is not stored anywhere, and after about an hour, |
| 153 | it becomes impossible to recover the key for decrypting intercepted |
| 154 | communications even if the machine is cracked into or physically |
| 155 | seized. A value of zero indicates that the key will never be regenerated. |
| 156 | .It Fl p Ar port |
| 157 | Specifies the port on which the server listens for connections |
| 158 | (default 22). |
| 159 | .It Fl q |
| 160 | Quiet mode. Nothing is sent to the system log. Normally the beginning, |
| 161 | authentication, and termination of each connection is logged. |
| 162 | .It Fl Q |
| 163 | Do not print an error message if RSA support is missing. |
| 164 | .El |
| 165 | .Sh CONFIGURATION FILE |
| 166 | .Nm |
| 167 | reads configuration data from |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 168 | .Pa /etc/ssh/sshd_config |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 169 | (or the file specified with |
| 170 | .Fl f |
| 171 | on the command line). The file |
| 172 | contains keyword-value pairs, one per line. Lines starting with |
| 173 | .Ql # |
| 174 | and empty lines are interpreted as comments. |
| 175 | .Pp |
| 176 | The following keywords are possible. |
| 177 | .Bl -tag -width Ds |
| 178 | .It Cm AFSTokenPassing |
| 179 | Specifies whether an AFS token may be forwarded to the server. Default is |
| 180 | .Dq yes . |
| 181 | .It Cm AllowGroups |
| 182 | This keyword can be followed by a number of group names, separated |
| 183 | by spaces. If specified, login is allowed only for users whose primary |
| 184 | group matches one of the patterns. |
| 185 | .Ql \&* |
| 186 | and |
| 187 | .Ql ? |
| 188 | can be used as |
| 189 | wildcards in the patterns. Only group names are valid, a numerical group |
| 190 | id isn't recognized. By default login is allowed regardless of |
| 191 | the primary group. |
| 192 | .Pp |
| 193 | .It Cm AllowUsers |
| 194 | This keyword can be followed by a number of user names, separated |
| 195 | by spaces. If specified, login is allowed only for users names that |
| 196 | match one of the patterns. |
| 197 | .Ql \&* |
| 198 | and |
| 199 | .Ql ? |
| 200 | can be used as |
| 201 | wildcards in the patterns. Only user names are valid, a numerical user |
| 202 | id isn't recognized. By default login is allowed regardless of |
| 203 | the user name. |
| 204 | .Pp |
| 205 | .It Cm CheckMail |
| 206 | Specifies whether |
| 207 | .Nm |
| 208 | should check for new mail for interactive logins. |
| 209 | The default is |
| 210 | .Dq no . |
| 211 | .It Cm DenyGroups |
| 212 | This keyword can be followed by a number of group names, separated |
| 213 | by spaces. Users whose primary group matches one of the patterns |
| 214 | aren't allowed to log in. |
| 215 | .Ql \&* |
| 216 | and |
| 217 | .Ql ? |
| 218 | can be used as |
| 219 | wildcards in the patterns. Only group names are valid, a numerical group |
| 220 | id isn't recognized. By default login is allowed regardless of |
| 221 | the primary group. |
| 222 | .Pp |
| 223 | .It Cm DenyUsers |
| 224 | This keyword can be followed by a number of user names, separated |
| 225 | by spaces. Login is allowed disallowed for user names that match |
| 226 | one of the patterns. |
| 227 | .Ql \&* |
| 228 | and |
| 229 | .Ql ? |
| 230 | can be used as |
| 231 | wildcards in the patterns. Only user names are valid, a numerical user |
| 232 | id isn't recognized. By default login is allowed regardless of |
| 233 | the user name. |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 234 | .It Cm HostKey |
| 235 | Specifies the file containing the private host key (default |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 236 | .Pa /etc/ssh/ssh_host_key ) . |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 237 | Note that |
| 238 | .Nm |
| 239 | does not start if this file is group/world-accessible. |
| 240 | .It Cm IgnoreRhosts |
| 241 | Specifies that rhosts and shosts files will not be used in |
| 242 | authentication. |
| 243 | .Pa /etc/hosts.equiv |
| 244 | and |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 245 | .Pa /etc/ssh/shosts.equiv |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 246 | are still used. The default is |
| 247 | .Dq no . |
Damien Miller | 3226509 | 1999-11-12 11:33:04 +1100 | [diff] [blame] | 248 | .It Cm IgnoreUserKnownHosts |
| 249 | Specifies whether |
| 250 | .Nm |
| 251 | should ignore the user's |
| 252 | .Pa $HOME/.ssh/known_hosts |
| 253 | during |
| 254 | .Cm RhostsRSAAuthentication . |
| 255 | The default is |
| 256 | .Dq no . |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 257 | .It Cm KeepAlive |
| 258 | Specifies whether the system should send keepalive messages to the |
| 259 | other side. If they are sent, death of the connection or crash of one |
| 260 | of the machines will be properly noticed. However, this means that |
| 261 | connections will die if the route is down temporarily, and some people |
| 262 | find it annoying. On the other hand, if keepalives are not send, |
| 263 | sessions may hang indefinitely on the server, leaving |
| 264 | .Dq ghost |
| 265 | users and consuming server resources. |
| 266 | .Pp |
| 267 | The default is |
| 268 | .Dq yes |
| 269 | (to send keepalives), and the server will notice |
| 270 | if the network goes down or the client host reboots. This avoids |
| 271 | infinitely hanging sessions. |
| 272 | .Pp |
| 273 | To disable keepalives, the value should be set to |
| 274 | .Dq no |
| 275 | in both the server and the client configuration files. |
| 276 | .It Cm KerberosAuthentication |
| 277 | Specifies whether Kerberos authentication is allowed. This can |
| 278 | be in the form of a Kerberos ticket, or if |
| 279 | .Cm PasswordAuthentication |
| 280 | is yes, the password provided by the user will be validated through |
| 281 | the Kerberos KDC. Default is |
| 282 | .Dq yes . |
| 283 | .It Cm KerberosOrLocalPasswd |
| 284 | If set then if password authentication through Kerberos fails then |
| 285 | the password will be validated via any additional local mechanism |
| 286 | such as |
| 287 | .Pa /etc/passwd |
| 288 | or SecurID. Default is |
| 289 | .Dq yes . |
| 290 | .It Cm KerberosTgtPassing |
| 291 | Specifies whether a Kerberos TGT may be forwarded to the server. |
| 292 | Default is |
| 293 | .Dq no , |
| 294 | as this only works when the Kerberos KDC is actually an AFS kaserver. |
| 295 | .It Cm KerberosTicketCleanup |
| 296 | Specifies whether to automatically destroy the user's ticket cache |
| 297 | file on logout. Default is |
| 298 | .Dq yes . |
| 299 | .It Cm KeyRegenerationInterval |
| 300 | The server key is automatically regenerated after this many seconds |
| 301 | (if it has been used). The purpose of regeneration is to prevent |
| 302 | decrypting captured sessions by later breaking into the machine and |
| 303 | stealing the keys. The key is never stored anywhere. If the value is |
| 304 | 0, the key is never regenerated. The default is 3600 |
| 305 | (seconds). |
| 306 | .It Cm ListenAddress |
| 307 | Specifies what local address |
| 308 | .Nm |
| 309 | should listen on. |
| 310 | The default is to listen to all local addresses. |
| 311 | .It Cm LoginGraceTime |
| 312 | The server disconnects after this time if the user has not |
| 313 | successfully logged in. If the value is 0, there is no time limit. |
| 314 | The default is 600 (seconds). |
Damien Miller | 5ce662a | 1999-11-11 17:57:39 +1100 | [diff] [blame] | 315 | .It Cm LogLevel |
| 316 | Gives the verbosity level that is used when logging messages from |
| 317 | .Nm sshd . |
| 318 | The possible values are: |
| 319 | QUIET, FATAL, ERROR, INFO, CHAT and DEBUG. |
| 320 | The default is INFO. |
| 321 | Logging with level DEBUG violates the privacy of users |
| 322 | and is not recommended. |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 323 | .It Cm PasswordAuthentication |
| 324 | Specifies whether password authentication is allowed. |
| 325 | The default is |
| 326 | .Dq yes . |
| 327 | .It Cm PermitEmptyPasswords |
| 328 | When password authentication is allowed, it specifies whether the |
| 329 | server allows login to accounts with empty password strings. The default |
| 330 | is |
| 331 | .Dq yes . |
| 332 | .It Cm PermitRootLogin |
| 333 | Specifies whether the root can log in using |
| 334 | .Xr ssh 1 . |
| 335 | The argument must be |
| 336 | .Dq yes , |
| 337 | .Dq without-password |
| 338 | or |
| 339 | .Dq no . |
| 340 | The default is |
| 341 | .Dq yes . |
| 342 | If this options is set to |
| 343 | .Dq without-password |
| 344 | only password authentication is disabled for root. |
| 345 | .Pp |
| 346 | Root login with RSA authentication when the |
| 347 | .Ar command |
| 348 | option has been |
| 349 | specified will be allowed regardless of the value of this setting |
| 350 | (which may be useful for taking remote backups even if root login is |
| 351 | normally not allowed). |
| 352 | .It Cm Port |
| 353 | Specifies the port number that |
| 354 | .Nm |
| 355 | listens on. The default is 22. |
| 356 | .It Cm PrintMotd |
| 357 | Specifies whether |
| 358 | .Nm |
| 359 | should print |
| 360 | .Pa /etc/motd |
| 361 | when a user logs in interactively. (On some systems it is also |
| 362 | printed by the shell, |
| 363 | .Pa /etc/profile , |
| 364 | or equivalent.) The default is |
| 365 | .Dq yes . |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 366 | .It Cm RandomSeed |
| 367 | Obsolete. Random number generation uses other techniques. |
| 368 | .It Cm RhostsAuthentication |
| 369 | Specifies whether authentication using rhosts or /etc/hosts.equiv |
| 370 | files is sufficient. Normally, this method should not be permitted |
| 371 | because it is insecure. |
| 372 | .Cm RhostsRSAAuthentication |
| 373 | should be used |
| 374 | instead, because it performs RSA-based host authentication in addition |
| 375 | to normal rhosts or /etc/hosts.equiv authentication. |
| 376 | The default is |
| 377 | .Dq no . |
| 378 | .It Cm RhostsRSAAuthentication |
| 379 | Specifies whether rhosts or /etc/hosts.equiv authentication together |
| 380 | with successful RSA host authentication is allowed. The default is |
| 381 | .Dq yes . |
| 382 | .It Cm RSAAuthentication |
| 383 | Specifies whether pure RSA authentication is allowed. The default is |
| 384 | .Dq yes . |
| 385 | .It Cm ServerKeyBits |
| 386 | Defines the number of bits in the server key. The minimum value is |
| 387 | 512, and the default is 768. |
| 388 | .It Cm SkeyAuthentication |
| 389 | Specifies whether |
| 390 | .Xr skey 1 |
| 391 | authentication is allowed. The default is |
| 392 | .Dq yes . |
| 393 | Note that s/key authentication is enabled only if |
| 394 | .Cm PasswordAuthentication |
| 395 | is allowed, too. |
| 396 | .It Cm StrictModes |
| 397 | Specifies whether |
| 398 | .Nm |
| 399 | should check file modes and ownership of the |
| 400 | user's files and home directory before accepting login. This |
| 401 | is normally desirable because novices sometimes accidentally leave their |
| 402 | directory or files world-writable. The default is |
| 403 | .Dq yes . |
| 404 | .It Cm SyslogFacility |
| 405 | Gives the facility code that is used when logging messages from |
| 406 | .Nm sshd . |
| 407 | The possible values are: DAEMON, USER, AUTH, LOCAL0, LOCAL1, LOCAL2, |
| 408 | LOCAL3, LOCAL4, LOCAL5, LOCAL6, LOCAL7. The default is AUTH. |
| 409 | .It Cm UseLogin |
| 410 | Specifies whether |
| 411 | .Xr login 1 |
| 412 | is used. The default is |
| 413 | .Dq no . |
| 414 | .It Cm X11Forwarding |
| 415 | Specifies whether X11 forwarding is permitted. The default is |
| 416 | .Dq yes . |
| 417 | Note that disabling X11 forwarding does not improve security in any |
| 418 | way, as users can always install their own forwarders. |
| 419 | .It Cm X11DisplayOffset |
| 420 | Specifies the first display number available for |
| 421 | .Nm sshd Ns 's |
| 422 | X11 forwarding. This prevents |
| 423 | .Nm |
| 424 | from interfering with real X11 servers. |
| 425 | .El |
| 426 | .Sh LOGIN PROCESS |
| 427 | When a user successfully logs in, |
| 428 | .Nm |
| 429 | does the following: |
| 430 | .Bl -enum -offset indent |
| 431 | .It |
| 432 | If the login is on a tty, and no command has been specified, |
| 433 | prints last login time and |
| 434 | .Pa /etc/motd |
| 435 | (unless prevented in the configuration file or by |
| 436 | .Pa $HOME/.hushlogin ; |
| 437 | see the |
| 438 | .Sx FILES |
| 439 | section). |
| 440 | .It |
| 441 | If the login is on a tty, records login time. |
| 442 | .It |
| 443 | Checks |
| 444 | .Pa /etc/nologin ; |
| 445 | if it exists, prints contents and quits |
| 446 | (unless root). |
| 447 | .It |
| 448 | Changes to run with normal user privileges. |
| 449 | .It |
| 450 | Sets up basic environment. |
| 451 | .It |
| 452 | Reads |
| 453 | .Pa $HOME/.ssh/environment |
| 454 | if it exists. |
| 455 | .It |
| 456 | Changes to user's home directory. |
| 457 | .It |
| 458 | If |
| 459 | .Pa $HOME/.ssh/rc |
| 460 | exists, runs it; else if |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 461 | .Pa /etc/ssh/sshrc |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 462 | exists, runs |
| 463 | it; otherwise runs xauth. The |
| 464 | .Dq rc |
| 465 | files are given the X11 |
| 466 | authentication protocol and cookie in standard input. |
| 467 | .It |
| 468 | Runs user's shell or command. |
| 469 | .El |
| 470 | .Sh AUTHORIZED_KEYS FILE FORMAT |
| 471 | The |
| 472 | .Pa $HOME/.ssh/authorized_keys |
| 473 | file lists the RSA keys that are |
| 474 | permitted for RSA authentication. Each line of the file contains one |
| 475 | key (empty lines and lines starting with a |
| 476 | .Ql # |
| 477 | are ignored as |
| 478 | comments). Each line consists of the following fields, separated by |
| 479 | spaces: options, bits, exponent, modulus, comment. The options field |
| 480 | is optional; its presence is determined by whether the line starts |
| 481 | with a number or not (the option field never starts with a number). |
| 482 | The bits, exponent, modulus and comment fields give the RSA key; the |
| 483 | comment field is not used for anything (but may be convenient for the |
| 484 | user to identify the key). |
| 485 | .Pp |
| 486 | Note that lines in this file are usually several hundred bytes long |
| 487 | (because of the size of the RSA key modulus). You don't want to type |
| 488 | them in; instead, copy the |
| 489 | .Pa identity.pub |
| 490 | file and edit it. |
| 491 | .Pp |
| 492 | The options (if present) consists of comma-separated option |
| 493 | specifications. No spaces are permitted, except within double quotes. |
| 494 | The following option specifications are supported: |
| 495 | .Bl -tag -width Ds |
| 496 | .It Cm from="pattern-list" |
| 497 | Specifies that in addition to RSA authentication, the canonical name |
| 498 | of the remote host must be present in the comma-separated list of |
| 499 | patterns ('*' and '?' serve as wildcards). The list may also contain |
| 500 | patterns negated by prefixing them with '!'; if the canonical host |
| 501 | name matches a negated pattern, the key is not accepted. The purpose |
| 502 | of this option is to optionally increase security: RSA authentication |
| 503 | by itself does not trust the network or name servers or anything (but |
| 504 | the key); however, if somebody somehow steals the key, the key |
| 505 | permits an intruder to log in from anywhere in the world. This |
| 506 | additional option makes using a stolen key more difficult (name |
| 507 | servers and/or routers would have to be compromised in addition to |
| 508 | just the key). |
| 509 | .It Cm command="command" |
| 510 | Specifies that the command is executed whenever this key is used for |
| 511 | authentication. The command supplied by the user (if any) is ignored. |
| 512 | The command is run on a pty if the connection requests a pty; |
| 513 | otherwise it is run without a tty. A quote may be included in the |
| 514 | command by quoting it with a backslash. This option might be useful |
| 515 | to restrict certain RSA keys to perform just a specific operation. An |
| 516 | example might be a key that permits remote backups but nothing |
| 517 | else. Notice that the client may specify TCP/IP and/or X11 |
| 518 | forwardings unless they are explicitly prohibited. |
| 519 | .It Cm environment="NAME=value" |
| 520 | Specifies that the string is to be added to the environment when |
| 521 | logging in using this key. Environment variables set this way |
| 522 | override other default environment values. Multiple options of this |
| 523 | type are permitted. |
| 524 | .It Cm no-port-forwarding |
| 525 | Forbids TCP/IP forwarding when this key is used for authentication. |
| 526 | Any port forward requests by the client will return an error. This |
| 527 | might be used, e.g., in connection with the |
| 528 | .Cm command |
| 529 | option. |
| 530 | .It Cm no-X11-forwarding |
| 531 | Forbids X11 forwarding when this key is used for authentication. |
| 532 | Any X11 forward requests by the client will return an error. |
| 533 | .It Cm no-agent-forwarding |
| 534 | Forbids authentication agent forwarding when this key is used for |
| 535 | authentication. |
| 536 | .It Cm no-pty |
| 537 | Prevents tty allocation (a request to allocate a pty will fail). |
| 538 | .El |
| 539 | .Ss Examples |
| 540 | 1024 33 12121.\|.\|.\|312314325 ylo@foo.bar |
| 541 | .Pp |
| 542 | from="*.niksula.hut.fi,!pc.niksula.hut.fi" 1024 35 23.\|.\|.\|2334 ylo@niksula |
| 543 | .Pp |
| 544 | command="dump /home",no-pty,no-port-forwarding 1024 33 23.\|.\|.\|2323 backup.hut.fi |
| 545 | .Sh SSH_KNOWN_HOSTS FILE FORMAT |
| 546 | The |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 547 | .Pa /etc/ssh/ssh_known_hosts |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 548 | and |
| 549 | .Pa $HOME/.ssh/known_hosts |
| 550 | files contain host public keys for all known hosts. The global file should |
| 551 | be prepared by the admistrator (optional), and the per-user file is |
| 552 | maintained automatically: whenever the user connects an unknown host |
| 553 | its key is added to the per-user file. |
| 554 | .Pp |
| 555 | Each line in these files contains the following fields: hostnames, |
| 556 | bits, exponent, modulus, comment. The fields are separated by spaces. |
| 557 | .Pp |
| 558 | Hostnames is a comma-separated list of patterns ('*' and '?' act as |
| 559 | wildcards); each pattern in turn is matched against the canonical host |
| 560 | name (when authenticating a client) or against the user-supplied |
| 561 | name (when authenticating a server). A pattern may also be preceded |
| 562 | by |
| 563 | .Ql ! |
| 564 | to indicate negation: if the host name matches a negated |
| 565 | pattern, it is not accepted (by that line) even if it matched another |
| 566 | pattern on the line. |
| 567 | .Pp |
| 568 | Bits, exponent, and modulus are taken directly from the host key; they |
| 569 | can be obtained, e.g., from |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 570 | .Pa /etc/ssh/ssh_host_key.pub . |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 571 | The optional comment field continues to the end of the line, and is not used. |
| 572 | .Pp |
| 573 | Lines starting with |
| 574 | .Ql # |
| 575 | and empty lines are ignored as comments. |
| 576 | .Pp |
| 577 | When performing host authentication, authentication is accepted if any |
| 578 | matching line has the proper key. It is thus permissible (but not |
| 579 | recommended) to have several lines or different host keys for the same |
| 580 | names. This will inevitably happen when short forms of host names |
| 581 | from different domains are put in the file. It is possible |
| 582 | that the files contain conflicting information; authentication is |
| 583 | accepted if valid information can be found from either file. |
| 584 | .Pp |
| 585 | Note that the lines in these files are typically hundreds of characters |
| 586 | long, and you definitely don't want to type in the host keys by hand. |
| 587 | Rather, generate them by a script |
| 588 | or by taking |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 589 | .Pa /etc/ssh/ssh_host_key.pub |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 590 | and adding the host names at the front. |
| 591 | .Ss Examples |
| 592 | closenet,closenet.hut.fi,.\|.\|.\|,130.233.208.41 1024 37 159.\|.\|.93 closenet.hut.fi |
| 593 | .Sh FILES |
| 594 | .Bl -tag -width Ds |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 595 | .It Pa /etc/ssh/sshd_config |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 596 | Contains configuration data for |
| 597 | .Nm sshd . |
| 598 | This file should be writable by root only, but it is recommended |
| 599 | (though not necessary) that it be world-readable. |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 600 | .It Pa /etc/ssh/ssh_host_key |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 601 | Contains the private part of the host key. |
| 602 | This file should only be owned by root, readable only by root, and not |
| 603 | accessible to others. |
| 604 | Note that |
| 605 | .Nm |
| 606 | does not start if this file is group/world-accessible. |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 607 | .It Pa /etc/ssh/ssh_host_key.pub |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 608 | Contains the public part of the host key. |
| 609 | This file should be world-readable but writable only by |
| 610 | root. Its contents should match the private part. This file is not |
| 611 | really used for anything; it is only provided for the convenience of |
| 612 | the user so its contents can be copied to known hosts files. |
| 613 | These two files are created using |
| 614 | .Xr ssh-keygen 1 . |
| 615 | .It Pa /var/run/sshd.pid |
| 616 | Contains the process ID of the |
| 617 | .Nm |
| 618 | listening for connections (if there are several daemons running |
| 619 | concurrently for different ports, this contains the pid of the one |
| 620 | started last). The contents of this file are not sensitive; it can be |
| 621 | world-readable. |
| 622 | .It Pa $HOME/.ssh/authorized_keys |
| 623 | Lists the RSA keys that can be used to log into the user's account. |
| 624 | This file must be readable by root (which may on some machines imply |
| 625 | it being world-readable if the user's home directory resides on an NFS |
| 626 | volume). It is recommended that it not be accessible by others. The |
| 627 | format of this file is described above. |
Damien Miller | 5ce662a | 1999-11-11 17:57:39 +1100 | [diff] [blame] | 628 | .It Pa "/etc/ssh_known_hosts" and "$HOME/.ssh/known_hosts" |
| 629 | These files are consulted when using rhosts with RSA host |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 630 | authentication to check the public key of the host. The key must be |
Damien Miller | 33e511e | 1999-11-11 11:43:13 +1100 | [diff] [blame] | 631 | listed in one of these files to be accepted. |
| 632 | The client uses the same files |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 633 | to verify that the remote host is the one we intended to |
| 634 | connect. These files should be writable only by root/the owner. |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 635 | .Pa /etc/ssh/ssh_known_hosts |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 636 | should be world-readable, and |
| 637 | .Pa $HOME/.ssh/known_hosts |
| 638 | can but need not be world-readable. |
| 639 | .It Pa /etc/nologin |
| 640 | If this file exists, |
| 641 | .Nm |
| 642 | refuses to let anyone except root log in. The contents of the file |
| 643 | are displayed to anyone trying to log in, and non-root connections are |
| 644 | refused. The file should be world-readable. |
| 645 | .It Pa /etc/hosts.allow, /etc/hosts.deny |
| 646 | If compiled with |
| 647 | .Sy LIBWRAP |
| 648 | support, tcp-wrappers access controls may be defined here as described in |
| 649 | .Xr hosts_access 5 . |
| 650 | .It Pa $HOME/.rhosts |
| 651 | This file contains host-username pairs, separated by a space, one per |
| 652 | line. The given user on the corresponding host is permitted to log in |
| 653 | without password. The same file is used by rlogind and rshd. |
| 654 | The file must |
| 655 | be writable only by the user; it is recommended that it not be |
| 656 | accessible by others. |
| 657 | .Pp |
| 658 | If is also possible to use netgroups in the file. Either host or user |
| 659 | name may be of the form +@groupname to specify all hosts or all users |
| 660 | in the group. |
| 661 | .It Pa $HOME/.shosts |
| 662 | For ssh, |
| 663 | this file is exactly the same as for |
| 664 | .Pa .rhosts . |
| 665 | However, this file is |
| 666 | not used by rlogin and rshd, so using this permits access using SSH only. |
| 667 | .Pa /etc/hosts.equiv |
| 668 | This file is used during |
| 669 | .Pa .rhosts |
| 670 | authentication. In the |
| 671 | simplest form, this file contains host names, one per line. Users on |
| 672 | those hosts are permitted to log in without a password, provided they |
| 673 | have the same user name on both machines. The host name may also be |
| 674 | followed by a user name; such users are permitted to log in as |
| 675 | .Em any |
| 676 | user on this machine (except root). Additionally, the syntax |
| 677 | .Dq +@group |
| 678 | can be used to specify netgroups. Negated entries start with |
| 679 | .Ql \&- . |
| 680 | .Pp |
| 681 | If the client host/user is successfully matched in this file, login is |
| 682 | automatically permitted provided the client and server user names are the |
| 683 | same. Additionally, successful RSA host authentication is normally |
| 684 | required. This file must be writable only by root; it is recommended |
| 685 | that it be world-readable. |
| 686 | .Pp |
| 687 | .Sy "Warning: It is almost never a good idea to use user names in" |
| 688 | .Pa hosts.equiv . |
| 689 | Beware that it really means that the named user(s) can log in as |
| 690 | .Em anybody , |
| 691 | which includes bin, daemon, adm, and other accounts that own critical |
| 692 | binaries and directories. Using a user name practically grants the |
| 693 | user root access. The only valid use for user names that I can think |
| 694 | of is in negative entries. |
| 695 | .Pp |
| 696 | Note that this warning also applies to rsh/rlogin. |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 697 | .It Pa /etc/ssh/shosts.equiv |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 698 | This is processed exactly as |
| 699 | .Pa /etc/hosts.equiv . |
| 700 | However, this file may be useful in environments that want to run both |
| 701 | rsh/rlogin and ssh. |
| 702 | .It Pa $HOME/.ssh/environment |
| 703 | This file is read into the environment at login (if it exists). It |
| 704 | can only contain empty lines, comment lines (that start with |
| 705 | .Ql # ) , |
| 706 | and assignment lines of the form name=value. The file should be writable |
| 707 | only by the user; it need not be readable by anyone else. |
| 708 | .It Pa $HOME/.ssh/rc |
| 709 | If this file exists, it is run with /bin/sh after reading the |
| 710 | environment files but before starting the user's shell or command. If |
| 711 | X11 spoofing is in use, this will receive the "proto cookie" pair in |
| 712 | standard input (and |
| 713 | .Ev DISPLAY |
| 714 | in environment). This must call |
| 715 | .Xr xauth 1 |
| 716 | in that case. |
| 717 | .Pp |
| 718 | The primary purpose of this file is to run any initialization routines |
| 719 | which may be needed before the user's home directory becomes |
| 720 | accessible; AFS is a particular example of such an environment. |
| 721 | .Pp |
| 722 | This file will probably contain some initialization code followed by |
| 723 | something similar to: "if read proto cookie; then echo add $DISPLAY |
| 724 | $proto $cookie | xauth -q -; fi". |
| 725 | .Pp |
| 726 | If this file does not exist, |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 727 | .Pa /etc/ssh/sshrc |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 728 | is run, and if that |
| 729 | does not exist either, xauth is used to store the cookie. |
| 730 | .Pp |
| 731 | This file should be writable only by the user, and need not be |
| 732 | readable by anyone else. |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 733 | .It Pa /etc/ssh/sshrc |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 734 | Like |
| 735 | .Pa $HOME/.ssh/rc . |
| 736 | This can be used to specify |
| 737 | machine-specific login-time initializations globally. This file |
| 738 | should be writable only by root, and should be world-readable. |
| 739 | .Sh AUTHOR |
| 740 | Tatu Ylonen <ylo@cs.hut.fi> |
| 741 | .Pp |
| 742 | Information about new releases, mailing lists, and other related |
| 743 | issues can be found from the SSH WWW home page: |
| 744 | .Pp |
| 745 | .Dl http://www.cs.hut.fi/ssh. |
| 746 | .Pp |
| 747 | OpenSSH |
| 748 | is a derivative of the original (free) ssh 1.2.12 release, but with bugs |
| 749 | removed and newer features re-added. Rapidly after the 1.2.12 release, |
| 750 | newer versions bore successively more restrictive licenses. This version |
| 751 | of OpenSSH |
| 752 | .Bl -bullet |
| 753 | .It |
| 754 | has all components of a restrictive nature (ie. patents, see |
| 755 | .Xr ssl 8 ) |
| 756 | directly removed from the source code; any licensed or patented components |
| 757 | are chosen from |
| 758 | external libraries. |
| 759 | .It |
| 760 | has been updated to support ssh protocol 1.5. |
| 761 | .It |
| 762 | contains added support for |
| 763 | .Xr kerberos 8 |
| 764 | authentication and ticket passing. |
| 765 | .It |
| 766 | supports one-time password authentication with |
| 767 | .Xr skey 1 . |
| 768 | .El |
| 769 | .Pp |
| 770 | The libraries described in |
| 771 | .Xr ssl 8 |
| 772 | are required for proper operation. |
| 773 | .Sh SEE ALSO |
| 774 | .Xr rlogin 1 , |
| 775 | .Xr rsh 1 , |
| 776 | .Xr scp 1 , |
| 777 | .Xr ssh 1 , |
| 778 | .Xr ssh-add 1 , |
| 779 | .Xr ssh-agent 1 , |
| 780 | .Xr ssh-keygen 1 , |
| 781 | .Xr ssl 8 |