Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 1 | .\" -*- nroff -*- |
| 2 | .\" |
| 3 | .\" ssh.1.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 | aae6c61 | 1999-12-06 11:47:28 +1100 | [diff] [blame] | 12 | .\" $Id: ssh.1,v 1.11 1999/12/06 00:47:29 damien Exp $ |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 13 | .\" |
| 14 | .Dd September 25, 1999 |
| 15 | .Dt SSH 1 |
| 16 | .Os |
| 17 | .Sh NAME |
| 18 | .Nm ssh |
| 19 | .Nd OpenSSH secure shell client (remote login program) |
| 20 | .Sh SYNOPSIS |
| 21 | .Nm ssh |
| 22 | .Op Fl l Ar login_name |
| 23 | .Op Ar hostname | user@hostname |
| 24 | .Op Ar command |
| 25 | .Pp |
| 26 | .Nm ssh |
| 27 | .Op Fl afgknqtvxCPX |
| 28 | .Op Fl c Ar blowfish | 3des |
| 29 | .Op Fl e Ar escape_char |
| 30 | .Op Fl i Ar identity_file |
| 31 | .Op Fl l Ar login_name |
| 32 | .Op Fl o Ar option |
| 33 | .Op Fl p Ar port |
| 34 | .Oo Fl L Xo |
| 35 | .Sm off |
| 36 | .Ar host : |
| 37 | .Ar port : |
| 38 | .Ar hostport |
| 39 | .Sm on |
| 40 | .Xc |
| 41 | .Oc |
| 42 | .Oo Fl R Xo |
| 43 | .Sm off |
| 44 | .Ar host : |
| 45 | .Ar port : |
| 46 | .Ar hostport |
| 47 | .Sm on |
| 48 | .Xc |
| 49 | .Oc |
| 50 | .Op Ar hostname | user@hostname |
| 51 | .Op Ar command |
| 52 | .Sh DESCRIPTION |
| 53 | .Nm |
| 54 | (Secure Shell) is a program for logging into a remote machine and for |
| 55 | executing commands on a remote machine. It is intended to replace |
| 56 | rlogin and rsh, and provide secure encrypted communications between |
| 57 | two untrusted hosts over an insecure network. X11 connections and |
| 58 | arbitrary TCP/IP ports can also be forwarded over the secure channel. |
| 59 | .Pp |
| 60 | .Nm |
| 61 | connects and logs into the specified |
| 62 | .Ar hostname . |
| 63 | The user must prove |
| 64 | his/her identity to the remote machine using one of several methods. |
| 65 | .Pp |
| 66 | First, if the machine the user logs in from is listed in |
| 67 | .Pa /etc/hosts.equiv |
| 68 | or |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 69 | .Pa /etc/ssh/shosts.equiv |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 70 | on the remote machine, and the user names are |
| 71 | the same on both sides, the user is immediately permitted to log in. |
| 72 | Second, if |
| 73 | .Pa \&.rhosts |
| 74 | or |
| 75 | .Pa \&.shosts |
| 76 | exists in the user's home directory on the |
| 77 | remote machine and contains a line containing the name of the client |
| 78 | machine and the name of the user on that machine, the user is |
| 79 | permitted to log in. This form of authentication alone is normally not |
| 80 | allowed by the server because it is not secure. |
| 81 | .Pp |
| 82 | The second (and primary) authentication method is the |
| 83 | .Pa rhosts |
| 84 | or |
| 85 | .Pa hosts.equiv |
| 86 | method combined with RSA-based host authentication. It |
| 87 | means that if the login would be permitted by |
| 88 | .Pa \&.rhosts , |
| 89 | .Pa \&.shosts , |
| 90 | .Pa /etc/hosts.equiv , |
| 91 | or |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 92 | .Pa /etc/ssh/shosts.equiv , |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 93 | and if additionally the server can verify the client's |
| 94 | host key (see |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 95 | .Pa /etc/ssh/ssh_known_hosts |
Damien Miller | 33e511e | 1999-11-11 11:43:13 +1100 | [diff] [blame] | 96 | and |
| 97 | .Pa $HOME/.ssh/known_hosts |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 98 | in the |
| 99 | .Sx FILES |
| 100 | section), only then login is |
| 101 | permitted. This authentication method closes security holes due to IP |
| 102 | spoofing, DNS spoofing and routing spoofing. [Note to the |
| 103 | administrator: |
| 104 | .Pa /etc/hosts.equiv , |
| 105 | .Pa \&.rhosts , |
| 106 | and the rlogin/rsh protocol in general, are inherently insecure and should be |
| 107 | disabled if security is desired.] |
| 108 | .Pp |
| 109 | As a third authentication method, |
| 110 | .Nm |
| 111 | supports RSA based authentication. |
| 112 | The scheme is based on public-key cryptography: there are cryptosystems |
| 113 | where encryption and decryption are done using separate keys, and it |
| 114 | is not possible to derive the decryption key from the encryption key. |
| 115 | RSA is one such system. The idea is that each user creates a public/private |
| 116 | key pair for authentication purposes. The |
| 117 | server knows the public key, and only the user knows the private key. |
| 118 | The file |
| 119 | .Pa $HOME/.ssh/authorized_keys |
| 120 | lists the public keys that are permitted for logging |
| 121 | in. When the user logs in, the |
| 122 | .Nm |
| 123 | program tells the server which key pair it would like to use for |
| 124 | authentication. The server checks if this key is permitted, and if |
| 125 | so, sends the user (actually the |
| 126 | .Nm |
| 127 | program running on behalf of the user) a challenge, a random number, |
| 128 | encrypted by the user's public key. The challenge can only be |
| 129 | decrypted using the proper private key. The user's client then decrypts the |
| 130 | challenge using the private key, proving that he/she knows the private |
| 131 | key but without disclosing it to the server. |
| 132 | .Pp |
| 133 | .Nm |
| 134 | implements the RSA authentication protocol automatically. The user |
| 135 | creates his/her RSA key pair by running |
| 136 | .Xr ssh-keygen 1 . |
| 137 | This stores the private key in |
| 138 | .Pa \&.ssh/identity |
| 139 | and the public key in |
| 140 | .Pa \&.ssh/identity.pub |
| 141 | in the user's home directory. The user should then |
| 142 | copy the |
| 143 | .Pa identity.pub |
| 144 | to |
| 145 | .Pa \&.ssh/authorized_keys |
| 146 | in his/her home directory on the remote machine (the |
| 147 | .Pa authorized_keys |
| 148 | file corresponds to the conventional |
| 149 | .Pa \&.rhosts |
| 150 | file, and has one key |
| 151 | per line, though the lines can be very long). After this, the user |
| 152 | can log in without giving the password. RSA authentication is much |
| 153 | more secure than rhosts authentication. |
| 154 | .Pp |
| 155 | The most convenient way to use RSA authentication may be with an |
| 156 | authentication agent. See |
| 157 | .Xr ssh-agent 1 |
| 158 | for more information. |
| 159 | .Pp |
| 160 | If other authentication methods fail, |
| 161 | .Nm |
| 162 | prompts the user for a password. The password is sent to the remote |
| 163 | host for checking; however, since all communications are encrypted, |
| 164 | the password cannot be seen by someone listening on the network. |
| 165 | .Pp |
| 166 | When the user's identity has been accepted by the server, the server |
| 167 | either executes the given command, or logs into the machine and gives |
| 168 | the user a normal shell on the remote machine. All communication with |
| 169 | the remote command or shell will be automatically encrypted. |
| 170 | .Pp |
| 171 | If a pseudo-terminal has been allocated (normal login session), the |
| 172 | user can disconnect with |
| 173 | .Ic ~. , |
| 174 | and suspend |
| 175 | .Nm |
| 176 | with |
| 177 | .Ic ~^Z . |
| 178 | All forwarded connections can be listed with |
| 179 | .Ic ~# |
| 180 | and if |
| 181 | the session blocks waiting for forwarded X11 or TCP/IP |
| 182 | connections to terminate, it can be backgrounded with |
| 183 | .Ic ~& |
| 184 | (this should not be used while the user shell is active, as it can cause the |
| 185 | shell to hang). All available escapes can be listed with |
| 186 | .Ic ~? . |
| 187 | .Pp |
| 188 | A single tilde character can be sent as |
| 189 | .Ic ~~ |
| 190 | (or by following the tilde by a character other than those described above). |
| 191 | The escape character must always follow a newline to be interpreted as |
| 192 | special. The escape character can be changed in configuration files |
| 193 | or on the command line. |
| 194 | .Pp |
| 195 | If no pseudo tty has been allocated, the |
| 196 | session is transparent and can be used to reliably transfer binary |
| 197 | data. On most systems, setting the escape character to |
| 198 | .Dq none |
| 199 | will also make the session transparent even if a tty is used. |
| 200 | .Pp |
| 201 | The session terminates when the command or shell in on the remote |
| 202 | machine exists and all X11 and TCP/IP connections have been closed. |
| 203 | The exit status of the remote program is returned as the exit status |
| 204 | of |
| 205 | .Nm ssh . |
| 206 | .Pp |
| 207 | If the user is using X11 (the |
| 208 | .Ev DISPLAY |
| 209 | environment variable is set), the connection to the X11 display is |
| 210 | automatically forwarded to the remote side in such a way that any X11 |
| 211 | programs started from the shell (or command) will go through the |
| 212 | encrypted channel, and the connection to the real X server will be made |
| 213 | from the local machine. The user should not manually set |
| 214 | .Ev DISPLAY . |
| 215 | Forwarding of X11 connections can be |
| 216 | configured on the command line or in configuration files. |
| 217 | .Pp |
| 218 | The |
| 219 | .Ev DISPLAY |
| 220 | value set by |
| 221 | .Nm |
| 222 | will point to the server machine, but with a display number greater |
| 223 | than zero. This is normal, and happens because |
| 224 | .Nm |
| 225 | creates a |
| 226 | .Dq proxy |
| 227 | X server on the server machine for forwarding the |
| 228 | connections over the encrypted channel. |
| 229 | .Pp |
| 230 | .Nm |
| 231 | will also automatically set up Xauthority data on the server machine. |
| 232 | For this purpose, it will generate a random authorization cookie, |
| 233 | store it in Xauthority on the server, and verify that any forwarded |
| 234 | connections carry this cookie and replace it by the real cookie when |
| 235 | the connection is opened. The real authentication cookie is never |
| 236 | sent to the server machine (and no cookies are sent in the plain). |
| 237 | .Pp |
| 238 | If the user is using an authentication agent, the connection to the agent |
| 239 | is automatically forwarded to the remote side unless disabled on |
| 240 | command line or in a configuration file. |
| 241 | .Pp |
| 242 | Forwarding of arbitrary TCP/IP connections over the secure channel can |
| 243 | be specified either on command line or in a configuration file. One |
| 244 | possible application of TCP/IP forwarding is a secure connection to an |
| 245 | electronic purse; another is going trough firewalls. |
| 246 | .Pp |
| 247 | .Nm |
| 248 | automatically maintains and checks a database containing RSA-based |
| 249 | identifications for all hosts it has ever been used with. The |
| 250 | database is stored in |
| 251 | .Pa \&.ssh/known_hosts |
| 252 | in the user's home directory. Additionally, the file |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 253 | .Pa /etc/ssh/ssh_known_hosts |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 254 | is automatically checked for known hosts. Any new hosts are |
| 255 | automatically added to the user's file. If a host's identification |
| 256 | ever changes, |
| 257 | .Nm |
| 258 | warns about this and disables password authentication to prevent a |
| 259 | trojan horse from getting the user's password. Another purpose of |
| 260 | this mechanism is to prevent man-in-the-middle attacks which could |
| 261 | otherwise be used to circumvent the encryption. The |
| 262 | .Cm StrictHostKeyChecking |
| 263 | option (see below) can be used to prevent logins to machines whose |
| 264 | host key is not known or has changed. |
| 265 | .Sh OPTIONS |
| 266 | .Bl -tag -width Ds |
| 267 | .It Fl a |
| 268 | Disables forwarding of the authentication agent connection. This may |
| 269 | also be specified on a per-host basis in the configuration file. |
| 270 | .It Fl c Ar blowfish|3des |
| 271 | Selects the cipher to use for encrypting the session. |
| 272 | .Ar 3des |
| 273 | is used by default. It is believed to be secure. |
| 274 | .Ar 3des |
| 275 | (triple-des) is an encrypt-decrypt-encrypt triple with three different keys. |
| 276 | It is presumably more secure than the |
| 277 | .Ar des |
| 278 | cipher which is no longer supported in ssh. |
| 279 | .Ar blowfish |
| 280 | is a fast block cipher, it appears very secure and is much faster than |
| 281 | .Ar 3des . |
| 282 | .It Fl e Ar ch|^ch|none |
| 283 | Sets the escape character for sessions with a pty (default: |
| 284 | .Ql ~ ) . |
| 285 | The escape character is only recognized at the beginning of a line. The |
| 286 | escape character followed by a dot |
| 287 | .Pq Ql \&. |
| 288 | closes the connection, followed |
| 289 | by control-Z suspends the connection, and followed by itself sends the |
| 290 | escape character once. Setting the character to |
| 291 | .Dq none |
| 292 | disables any escapes and makes the session fully transparent. |
| 293 | .It Fl f |
| 294 | Requests |
| 295 | .Nm |
Damien Miller | 5428f64 | 1999-11-25 11:54:57 +1100 | [diff] [blame] | 296 | to go to background just before command execution. This is useful |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 297 | if |
| 298 | .Nm |
| 299 | is going to ask for passwords or passphrases, but the user |
| 300 | wants it in the background. This implies |
| 301 | .Fl n . |
| 302 | The recommended way to start X11 programs at a remote site is with |
| 303 | something like |
| 304 | .Ic ssh -f host xterm . |
| 305 | .It Fl i Ar identity_file |
| 306 | Selects the file from which the identity (private key) for |
| 307 | RSA authentication is read. Default is |
| 308 | .Pa \&.ssh/identity |
| 309 | in the user's home directory. Identity files may also be specified on |
| 310 | a per-host basis in the configuration file. It is possible to have |
| 311 | multiple |
| 312 | .Fl i |
| 313 | options (and multiple identities specified in |
| 314 | configuration files). |
| 315 | .It Fl g |
| 316 | Allows remote hosts to connect to local forwarded ports. |
| 317 | .It Fl k |
| 318 | Disables forwarding of Kerberos tickets and AFS tokens. This may |
| 319 | also be specified on a per-host basis in the configuration file. |
| 320 | .It Fl l Ar login_name |
| 321 | Specifies the user to log in as on the remote machine. This may also |
| 322 | be specified on a per-host basis in the configuration file. |
| 323 | .It Fl n |
| 324 | Redirects stdin from |
| 325 | .Pa /dev/null |
| 326 | (actually, prevents reading from stdin). |
| 327 | This must be used when |
| 328 | .Nm |
| 329 | is run in the background. A common trick is to use this to run X11 |
| 330 | programs in a remote machine. For example, |
| 331 | .Ic ssh -n shadows.cs.hut.fi emacs & |
| 332 | will start an emacs on shadows.cs.hut.fi, and the X11 |
| 333 | connection will be automatically forwarded over an encrypted channel. |
| 334 | The |
| 335 | .Nm |
| 336 | program will be put in the background. |
| 337 | (This does not work if |
| 338 | .Nm |
| 339 | needs to ask for a password or passphrase; see also the |
| 340 | .Fl f |
| 341 | option.) |
| 342 | .It Fl o Ar option |
| 343 | Can be used to give options in the format used in the config file. |
| 344 | This is useful for specifying options for which there is no separate |
| 345 | command-line flag. The option has the same format as a line in the |
| 346 | configuration file. |
| 347 | .It Fl p Ar port |
| 348 | Port to connect to on the remote host. This can be specified on a |
| 349 | per-host basis in the configuration file. |
| 350 | .It Fl P |
| 351 | Use a non-privileged port for outgoing connections. |
| 352 | This can be used if your firewall does |
| 353 | not permit connections from privileged ports. |
| 354 | Note that this option turns of |
| 355 | .Cm RhostsAuthentication |
| 356 | and |
| 357 | .Cm RhostsRSAAuthentication . |
| 358 | .It Fl q |
| 359 | Quiet mode. Causes all warning and diagnostic messages to be |
| 360 | suppressed. Only fatal errors are displayed. |
| 361 | .It Fl t |
| 362 | Force pseudo-tty allocation. This can be used to execute arbitary |
| 363 | screen-based programs on a remote machine, which can be very useful |
| 364 | e.g. when implementing menu services. |
| 365 | .It Fl v |
| 366 | Verbose mode. Causes |
| 367 | .Nm |
| 368 | to print debugging messages about its progress. This is helpful in |
| 369 | debugging connection, authentication, and configuration problems. |
| 370 | The verbose mode is also used to display |
| 371 | .Xr skey 1 |
| 372 | challenges, if the user entered "s/key" as password. |
| 373 | .It Fl x |
| 374 | Disables X11 forwarding. This can also be specified on a per-host |
| 375 | basis in a configuration file. |
| 376 | .It Fl X |
| 377 | Enables X11 forwarding. |
| 378 | .It Fl C |
| 379 | Requests compression of all data (including stdin, stdout, stderr, and |
| 380 | data for forwarded X11 and TCP/IP connections). The compression |
| 381 | algorithm is the same used by gzip, and the |
| 382 | .Dq level |
| 383 | can be controlled by the |
| 384 | .Cm CompressionLevel |
| 385 | option (see below). Compression is desirable on modem lines and other |
| 386 | slow connections, but will only slow down things on fast networks. |
| 387 | The default value can be set on a host-by-host basis in the |
| 388 | configuration files; see the |
| 389 | .Cm Compress |
| 390 | option below. |
| 391 | .It Fl L Ar port:host:hostport |
| 392 | Specifies that the given port on the local (client) host is to be |
| 393 | forwarded to the given host and port on the remote side. This works |
| 394 | by allocating a socket to listen to |
| 395 | .Ar port |
| 396 | on the local side, and whenever a connection is made to this port, the |
| 397 | connection is forwarded over the secure channel, and a connection is |
| 398 | made to |
| 399 | .Ar host:hostport |
| 400 | from the remote machine. Port forwardings can also be specified in the |
| 401 | configuration file. Only root can forward privileged ports. |
| 402 | .It Fl R Ar port:host:hostport |
| 403 | Specifies that the given port on the remote (server) host is to be |
| 404 | forwarded to the given host and port on the local side. This works |
| 405 | by allocating a socket to listen to |
| 406 | .Ar port |
| 407 | on the remote side, and whenever a connection is made to this port, the |
| 408 | connection is forwarded over the secure channel, and a connection is |
| 409 | made to |
| 410 | .Ar host:hostport |
| 411 | from the local machine. Port forwardings can also be specified in the |
| 412 | configuration file. Privileged ports can be forwarded only when |
| 413 | logging in as root on the remote machine. |
| 414 | .El |
| 415 | .Sh CONFIGURATION FILES |
| 416 | .Nm |
| 417 | obtains configuration data from the following sources (in this order): |
| 418 | command line options, user's configuration file |
| 419 | .Pq Pa $HOME/.ssh/config , |
| 420 | and system-wide configuration file |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 421 | .Pq Pa /etc/ssh/ssh_config . |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 422 | For each parameter, the first obtained value |
| 423 | will be used. The configuration files contain sections bracketed by |
| 424 | "Host" specifications, and that section is only applied for hosts that |
| 425 | match one of the patterns given in the specification. The matched |
| 426 | host name is the one given on the command line. |
| 427 | .Pp |
| 428 | Since the first obtained value for each parameter is used, more |
| 429 | host-specific declarations should be given near the beginning of the |
| 430 | file, and general defaults at the end. |
| 431 | .Pp |
| 432 | The configuration file has the following format: |
| 433 | .Pp |
| 434 | Empty lines and lines starting with |
| 435 | .Ql # |
| 436 | are comments. |
| 437 | .Pp |
| 438 | Otherwise a line is of the format |
| 439 | .Dq keyword arguments . |
| 440 | The possible |
| 441 | keywords and their meanings are as follows (note that the |
| 442 | configuration files are case-sensitive): |
| 443 | .Bl -tag -width Ds |
| 444 | .It Cm Host |
| 445 | Restricts the following declarations (up to the next |
| 446 | .Cm Host |
| 447 | keyword) to be only for those hosts that match one of the patterns |
| 448 | given after the keyword. |
| 449 | .Ql \&* |
| 450 | and |
| 451 | .Ql ? |
| 452 | can be used as wildcards in the |
| 453 | patterns. A single |
| 454 | .Ql \&* |
| 455 | as a pattern can be used to provide global |
| 456 | defaults for all hosts. The host is the |
| 457 | .Ar hostname |
| 458 | argument given on the command line (i.e., the name is not converted to |
| 459 | a canonicalized host name before matching). |
| 460 | .It Cm AFSTokenPassing |
| 461 | Specifies whether to pass AFS tokens to remote host. The argument to |
| 462 | this keyword must be |
| 463 | .Dq yes |
| 464 | or |
| 465 | .Dq no . |
| 466 | .It Cm BatchMode |
| 467 | If set to |
| 468 | .Dq yes , |
| 469 | passphrase/password querying will be disabled. This |
| 470 | option is useful in scripts and other batch jobs where you have no |
| 471 | user to supply the password. The argument must be |
| 472 | .Dq yes |
| 473 | or |
| 474 | .Dq no . |
| 475 | .It Cm Cipher |
| 476 | Specifies the cipher to use for encrypting the session. Currently, |
| 477 | .Dq blowfish , |
| 478 | and |
| 479 | .Dq 3des |
| 480 | are supported. The default is |
| 481 | .Dq 3des . |
| 482 | .It Cm Compression |
| 483 | Specifies whether to use compression. The argument must be |
| 484 | .Dq yes |
| 485 | or |
| 486 | .Dq no . |
| 487 | .It Cm CompressionLevel |
| 488 | Specifies the compression level to use if compression is enable. The |
| 489 | argument must be an integer from 1 (fast) to 9 (slow, best). The |
| 490 | default level is 6, which is good for most applications. The meaning |
| 491 | of the values is the same as in GNU GZIP. |
| 492 | .It Cm ConnectionAttempts |
| 493 | Specifies the number of tries (one per second) to make before falling |
| 494 | back to rsh or exiting. The argument must be an integer. This may be |
| 495 | useful in scripts if the connection sometimes fails. |
| 496 | .It Cm EscapeChar |
| 497 | Sets the escape character (default: |
| 498 | .Ql ~ ) . |
| 499 | The escape character can also |
| 500 | be set on the command line. The argument should be a single |
| 501 | character, |
| 502 | .Ql ^ |
| 503 | followed by a letter, or |
| 504 | .Dq none |
| 505 | to disable the escape |
| 506 | character entirely (making the connection transparent for binary |
| 507 | data). |
| 508 | .It Cm FallBackToRsh |
| 509 | Specifies that if connecting via |
| 510 | .Nm |
| 511 | fails due to a connection refused error (there is no |
| 512 | .Xr sshd 8 |
| 513 | listening on the remote host), |
| 514 | .Xr rsh 1 |
| 515 | should automatically be used instead (after a suitable warning about |
| 516 | the session being unencrypted). The argument must be |
| 517 | .Dq yes |
| 518 | or |
| 519 | .Dq no . |
| 520 | .It Cm ForwardAgent |
| 521 | Specifies whether the connection to the authentication agent (if any) |
| 522 | will be forwarded to the remote machine. The argument must be |
| 523 | .Dq yes |
| 524 | or |
| 525 | .Dq no . |
| 526 | .It Cm ForwardX11 |
| 527 | Specifies whether X11 connections will be automatically redirected |
| 528 | over the secure channel and |
| 529 | .Ev DISPLAY |
| 530 | set. The argument must be |
| 531 | .Dq yes |
| 532 | or |
| 533 | .Dq no . |
| 534 | .It Cm GatewayPorts |
| 535 | Specifies whether remote hosts are allowed to connect to local |
| 536 | forwarded ports. |
| 537 | The argument must be |
| 538 | .Dq yes |
| 539 | or |
| 540 | .Dq no . |
| 541 | The default is |
| 542 | .Dq no . |
| 543 | .It Cm GlobalKnownHostsFile |
| 544 | Specifies a file to use instead of |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 545 | .Pa /etc/ssh/ssh_known_hosts . |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 546 | .It Cm HostName |
| 547 | Specifies the real host name to log into. This can be used to specify |
| 548 | nicnames or abbreviations for hosts. Default is the name given on the |
| 549 | command line. Numeric IP addresses are also permitted (both on the |
| 550 | command line and in |
| 551 | .Cm HostName |
| 552 | specifications). |
| 553 | .It Cm IdentityFile |
| 554 | Specifies the file from which the user's RSA authentication identity |
| 555 | is read (default |
| 556 | .Pa .ssh/identity |
| 557 | in the user's home directory). |
| 558 | Additionally, any identities represented by the authentication agent |
| 559 | will be used for authentication. The file name may use the tilde |
| 560 | syntax to refer to a user's home directory. It is possible to have |
| 561 | multiple identity files specified in configuration files; all these |
| 562 | identities will be tried in sequence. |
| 563 | .It Cm KeepAlive |
| 564 | Specifies whether the system should send keepalive messages to the |
| 565 | other side. If they are sent, death of the connection or crash of one |
| 566 | of the machines will be properly noticed. However, this means that |
| 567 | connections will die if the route is down temporarily, and some people |
| 568 | find it annoying. |
| 569 | .Pp |
| 570 | The default is |
| 571 | .Dq yes |
| 572 | (to send keepalives), and the client will notice |
| 573 | if the network goes down or the remote host dies. This is important |
| 574 | in scripts, and many users want it too. |
| 575 | .Pp |
| 576 | To disable keepalives, the value should be set to |
| 577 | .Dq no |
| 578 | in both the server and the client configuration files. |
| 579 | .It Cm KerberosAuthentication |
| 580 | Specifies whether Kerberos authentication will be used. The argument to |
| 581 | this keyword must be |
| 582 | .Dq yes |
| 583 | or |
| 584 | .Dq no . |
| 585 | .It Cm KerberosTgtPassing |
| 586 | Specifies whether a Kerberos TGT will be forwarded to the server. This |
| 587 | will only work if the Kerberos server is actually an AFS kaserver. The |
| 588 | argument to this keyword must be |
| 589 | .Dq yes |
| 590 | or |
| 591 | .Dq no . |
| 592 | .It Cm LocalForward |
| 593 | Specifies that a TCP/IP port on the local machine be forwarded over |
| 594 | the secure channel to given host:port from the remote machine. The |
| 595 | first argument must be a port number, and the second must be |
| 596 | host:port. Multiple forwardings may be specified, and additional |
| 597 | forwardings can be given on the command line. Only the root can |
| 598 | forward privileged ports. |
| 599 | .It Cm PasswordAuthentication |
| 600 | Specifies whether to use password authentication. The argument to |
| 601 | this keyword must be |
| 602 | .Dq yes |
| 603 | or |
| 604 | .Dq no . |
Damien Miller | 5ce662a | 1999-11-11 17:57:39 +1100 | [diff] [blame] | 605 | .It Cm LogLevel |
| 606 | Gives the verbosity level that is used when logging messages from |
| 607 | .Nm ssh . |
| 608 | The possible values are: |
| 609 | QUIET, FATAL, ERROR, INFO, CHAT and DEBUG. |
| 610 | The default is INFO. |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 611 | .It Cm NumberOfPasswordPrompts |
| 612 | Specifies the number of password prompts before giving up. The |
| 613 | argument to this keyword must be an integer. Default is 3. |
| 614 | .It Cm Port |
| 615 | Specifies the port number to connect on the remote host. Default is |
| 616 | 22. |
| 617 | .It Cm ProxyCommand |
| 618 | Specifies the command to use to connect to the server. The command |
| 619 | string extends to the end of the line, and is executed with /bin/sh. |
| 620 | In the command string, %h will be substituted by the host name to |
| 621 | connect and %p by the port. The command can be basically anything, |
| 622 | and should read from its stdin and write to its stdout. It should |
| 623 | eventually connect an |
| 624 | .Xr sshd 8 |
| 625 | server running on some machine, or execute |
| 626 | .Ic sshd -i |
| 627 | somewhere. Host key management will be done using the |
| 628 | HostName of the host being connected (defaulting to the name typed by |
| 629 | the user). |
Damien Miller | aae6c61 | 1999-12-06 11:47:28 +1100 | [diff] [blame] | 630 | Note that |
| 631 | .Cm CheckHostIP |
| 632 | is not available for connects with a proxy command. |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 633 | .Pp |
| 634 | .It Cm RemoteForward |
| 635 | Specifies that a TCP/IP port on the remote machine be forwarded over |
| 636 | the secure channel to given host:port from the local machine. The |
| 637 | first argument must be a port number, and the second must be |
| 638 | host:port. Multiple forwardings may be specified, and additional |
| 639 | forwardings can be given on the command line. Only the root can |
| 640 | forward privileged ports. |
| 641 | .It Cm RhostsAuthentication |
| 642 | Specifies whether to try rhosts based authentication. Note that this |
| 643 | declaration only affects the client side and has no effect whatsoever |
| 644 | on security. Disabling rhosts authentication may reduce |
| 645 | authentication time on slow connections when rhosts authentication is |
| 646 | not used. Most servers do not permit RhostsAuthentication because it |
| 647 | is not secure (see RhostsRSAAuthentication). The argument to this |
| 648 | keyword must be |
| 649 | .Dq yes |
| 650 | or |
| 651 | .Dq no . |
| 652 | .It Cm RhostsRSAAuthentication |
| 653 | Specifies whether to try rhosts based authentication with RSA host |
| 654 | authentication. This is the primary authentication method for most |
| 655 | sites. The argument must be |
| 656 | .Dq yes |
| 657 | or |
| 658 | .Dq no . |
| 659 | .It Cm RSAAuthentication |
| 660 | Specifies whether to try RSA authentication. The argument to this |
| 661 | keyword must be |
| 662 | .Dq yes |
| 663 | or |
| 664 | .Dq no . |
| 665 | RSA authentication will only be |
| 666 | attempted if the identity file exists, or an authentication agent is |
| 667 | running. |
Damien Miller | 95def09 | 1999-11-25 00:26:21 +1100 | [diff] [blame] | 668 | .It Cm SkeyAuthentication |
| 669 | Specifies whether to use |
| 670 | .Xr skey 1 |
| 671 | authentication. The argument to |
| 672 | this keyword must be |
| 673 | .Dq yes |
| 674 | or |
| 675 | .Dq no . |
| 676 | The default is |
| 677 | .Dq no . |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 678 | .It Cm CheckHostIP |
| 679 | If this flag is set to |
| 680 | .Dq yes , |
| 681 | ssh will additionally check the host ip address in the |
| 682 | .Pa known_hosts |
| 683 | file. This allows ssh to detect if a host key changed due to DNS spoofing. |
| 684 | If the option is set to |
| 685 | .Dq no , |
| 686 | the check will not be executed. |
| 687 | .It Cm StrictHostKeyChecking |
| 688 | If this flag is set to |
| 689 | .Dq yes , |
| 690 | .Nm |
| 691 | ssh will never automatically add host keys to the |
| 692 | .Pa $HOME/.ssh/known_hosts |
| 693 | file, and refuses to connect hosts whose host key has changed. This |
| 694 | provides maximum protection against trojan horse attacks. However, it |
| 695 | can be somewhat annoying if you don't have good |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 696 | .Pa /etc/ssh/ssh_known_hosts |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 697 | files installed and frequently |
| 698 | connect new hosts. Basically this option forces the user to manually |
| 699 | add any new hosts. Normally this option is disabled, and new hosts |
| 700 | will automatically be added to the known host files. The host keys of |
| 701 | known hosts will be verified automatically in either case. The |
| 702 | argument must be |
| 703 | .Dq yes |
| 704 | or |
| 705 | .Dq no . |
| 706 | .It Cm User |
| 707 | Specifies the user to log in as. This can be useful if you have a |
| 708 | different user name in different machines. This saves the trouble of |
| 709 | having to remember to give the user name on the command line. |
| 710 | .It Cm UserKnownHostsFile |
| 711 | Specifies a file to use instead of |
| 712 | .Pa $HOME/.ssh/known_hosts . |
| 713 | .It Cm UsePrivilegedPort |
| 714 | Specifies whether to use a privileged port for outgoing connections. |
| 715 | The argument must be |
| 716 | .Dq yes |
| 717 | or |
| 718 | .Dq no . |
| 719 | The default is |
| 720 | .Dq yes . |
| 721 | Note that setting this option to |
| 722 | .Dq no |
| 723 | turns of |
| 724 | .Cm RhostsAuthentication |
| 725 | and |
| 726 | .Cm RhostsRSAAuthentication . |
| 727 | .It Cm UseRsh |
| 728 | Specifies that rlogin/rsh should be used for this host. It is |
| 729 | possible that the host does not at all support the |
| 730 | .Nm |
| 731 | protocol. This causes |
| 732 | .Nm |
| 733 | to immediately exec |
| 734 | .Xr rsh 1 . |
| 735 | All other options (except |
| 736 | .Cm HostName ) |
| 737 | are ignored if this has been specified. The argument must be |
| 738 | .Dq yes |
| 739 | or |
| 740 | .Dq no . |
| 741 | .Sh ENVIRONMENT |
| 742 | .Nm |
| 743 | will normally set the following environment variables: |
| 744 | .Bl -tag -width Ds |
| 745 | .It Ev DISPLAY |
| 746 | The |
| 747 | .Ev DISPLAY |
| 748 | variable indicates the location of the X11 server. It is |
| 749 | automatically set by |
| 750 | .Nm |
| 751 | to point to a value of the form |
| 752 | .Dq hostname:n |
| 753 | where hostname indicates |
| 754 | the host where the shell runs, and n is an integer >= 1. Ssh uses |
| 755 | this special value to forward X11 connections over the secure |
| 756 | channel. The user should normally not set DISPLAY explicitly, as that |
| 757 | will render the X11 connection insecure (and will require the user to |
| 758 | manually copy any required authorization cookies). |
| 759 | .It Ev HOME |
| 760 | Set to the path of the user's home directory. |
| 761 | .It Ev LOGNAME |
| 762 | Synonym for |
| 763 | .Ev USER ; |
| 764 | set for compatibility with systems that use this variable. |
| 765 | .It Ev MAIL |
| 766 | Set to point the user's mailbox. |
| 767 | .It Ev PATH |
| 768 | Set to the default |
| 769 | .Ev PATH , |
| 770 | as specified when compiling |
| 771 | .Nm ssh . |
| 772 | .It Ev SSH_AUTH_SOCK |
| 773 | indicates the path of a unix-domain socket used to communicate with the |
| 774 | agent. |
| 775 | .It Ev SSH_CLIENT |
| 776 | Identifies the client end of the connection. The variable contains |
| 777 | three space-separated values: client ip-address, client port number, |
| 778 | and server port number. |
| 779 | .It Ev SSH_TTY |
| 780 | This is set to the name of the tty (path to the device) associated |
| 781 | with the current shell or command. If the current session has no tty, |
| 782 | this variable is not set. |
| 783 | .It Ev TZ |
| 784 | The timezone variable is set to indicate the present timezone if it |
| 785 | was set when the daemon was started (e.i., the daemon passes the value |
| 786 | on to new connections). |
| 787 | .It Ev USER |
| 788 | Set to the name of the user logging in. |
| 789 | .El |
| 790 | .Pp |
| 791 | Additionally, |
| 792 | .Nm |
| 793 | reads |
| 794 | .Pa $HOME/.ssh/environment , |
| 795 | and adds lines of the format |
| 796 | .Dq VARNAME=value |
| 797 | to the environment. |
| 798 | .Sh FILES |
| 799 | .Bl -tag -width $HOME/.ssh/known_hosts |
| 800 | .It Pa $HOME/.ssh/known_hosts |
| 801 | Records host keys for all hosts the user has logged into (that are not |
| 802 | in |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 803 | .Pa /etc/ssh/ssh_known_hosts ) . |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 804 | See |
| 805 | .Xr sshd 8 . |
| 806 | .It Pa $HOME/.ssh/random_seed |
| 807 | Used for seeding the random number generator. This file contains |
| 808 | sensitive data and should read/write for the user and not accessible |
| 809 | for others. This file is created the first time the program is run |
| 810 | and updated automatically. The user should never need to read or |
| 811 | modify this file. |
| 812 | .It Pa $HOME/.ssh/identity |
| 813 | Contains the RSA authentication identity of the user. This file |
| 814 | contains sensitive data and should be readable by the user but not |
| 815 | accessible by others (read/write/execute). |
| 816 | Note that |
| 817 | .Nm |
| 818 | ignores this file if it is accessible by others. |
| 819 | It is possible to specify a passphrase when |
| 820 | generating the key; the passphrase will be used to encrypt the |
| 821 | sensitive part of this file using 3DES. |
| 822 | .It Pa $HOME/.ssh/identity.pub |
| 823 | Contains the public key for authentication (public part of the |
| 824 | identity file in human-readable form). The contents of this file |
| 825 | should be added to |
| 826 | .Pa $HOME/.ssh/authorized_keys |
| 827 | on all machines |
| 828 | where you wish to log in using RSA authentication. This file is not |
| 829 | sensitive and can (but need not) be readable by anyone. This file is |
| 830 | never used automatically and is not necessary; it is only provided for |
| 831 | the convenience of the user. |
| 832 | .It Pa $HOME/.ssh/config |
| 833 | This is the per-user configuration file. The format of this file is |
| 834 | described above. This file is used by the |
| 835 | .Nm |
| 836 | client. This file does not usually contain any sensitive information, |
| 837 | but the recommended permissions are read/write for the user, and not |
| 838 | accessible by others. |
| 839 | .It Pa $HOME/.ssh/authorized_keys |
| 840 | Lists the RSA keys that can be used for logging in as this user. The |
| 841 | format of this file is described in the |
| 842 | .Xr sshd 8 |
| 843 | manual page. In the simplest form the format is the same as the .pub |
| 844 | identity files (that is, each line contains the number of bits in |
| 845 | modulus, public exponent, modulus, and comment fields, separated by |
| 846 | spaces). This file is not highly sensitive, but the recommended |
| 847 | permissions are read/write for the user, and not accessible by others. |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 848 | .It Pa /etc/ssh/ssh_known_hosts |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 849 | Systemwide list of known host keys. This file should be prepared by the |
| 850 | system administrator to contain the public host keys of all machines in the |
| 851 | organization. This file should be world-readable. This file contains |
| 852 | public keys, one per line, in the following format (fields separated |
| 853 | by spaces): system name, number of bits in modulus, public exponent, |
| 854 | modulus, and optional comment field. When different names are used |
| 855 | for the same machine, all such names should be listed, separated by |
| 856 | commas. The format is described on the |
| 857 | .Xr sshd 8 |
| 858 | manual page. |
| 859 | .Pp |
| 860 | The canonical system name (as returned by name servers) is used by |
| 861 | .Xr sshd 8 |
| 862 | to verify the client host when logging in; other names are needed because |
| 863 | .Nm |
| 864 | does not convert the user-supplied name to a canonical name before |
| 865 | checking the key, because someone with access to the name servers |
| 866 | would then be able to fool host authentication. |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 867 | .It Pa /etc/ssh/ssh_config |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 868 | Systemwide configuration file. This file provides defaults for those |
| 869 | values that are not specified in the user's configuration file, and |
| 870 | for those users who do not have a configuration file. This file must |
| 871 | be world-readable. |
| 872 | .It Pa $HOME/.rhosts |
| 873 | This file is used in |
| 874 | .Pa \&.rhosts |
| 875 | authentication to list the |
| 876 | host/user pairs that are permitted to log in. (Note that this file is |
| 877 | also used by rlogin and rsh, which makes using this file insecure.) |
| 878 | Each line of the file contains a host name (in the canonical form |
| 879 | returned by name servers), and then a user name on that host, |
| 880 | separated by a space. One some machines this file may need to be |
| 881 | world-readable if the user's home directory is on a NFS partition, |
| 882 | because |
| 883 | .Xr sshd 8 |
| 884 | reads it as root. Additionally, this file must be owned by the user, |
| 885 | and must not have write permissions for anyone else. The recommended |
| 886 | permission for most machines is read/write for the user, and not |
| 887 | accessible by others. |
| 888 | .Pp |
| 889 | Note that by default |
| 890 | .Xr sshd 8 |
| 891 | will be installed so that it requires successful RSA host |
| 892 | authentication before permitting \s+2.\s0rhosts authentication. If your |
| 893 | server machine does not have the client's host key in |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 894 | .Pa /etc/ssh/ssh_known_hosts , |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 895 | you can store it in |
| 896 | .Pa $HOME/.ssh/known_hosts . |
| 897 | The easiest way to do this is to |
| 898 | connect back to the client from the server machine using ssh; this |
| 899 | will automatically add the host key inxi |
| 900 | .Pa $HOME/.ssh/known_hosts . |
| 901 | .It Pa $HOME/.shosts |
| 902 | This file is used exactly the same way as |
| 903 | .Pa \&.rhosts . |
| 904 | The purpose for |
| 905 | having this file is to be able to use rhosts authentication with |
| 906 | .Nm |
| 907 | without permitting login with |
| 908 | .Xr rlogin 1 |
| 909 | or |
| 910 | .Xr rsh 1 . |
| 911 | .It Pa /etc/hosts.equiv |
| 912 | This file is used during |
| 913 | .Pa \&.rhosts authentication. It contains |
| 914 | canonical hosts names, one per line (the full format is described on |
| 915 | the |
| 916 | .Xr sshd 8 |
| 917 | manual page). If the client host is found in this file, login is |
| 918 | automatically permitted provided client and server user names are the |
| 919 | same. Additionally, successful RSA host authentication is normally |
| 920 | required. This file should only be writable by root. |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 921 | .It Pa /etc/ssh/shosts.equiv |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 922 | This file is processed exactly as |
| 923 | .Pa /etc/hosts.equiv . |
| 924 | This file may be useful to permit logins using |
| 925 | .Nm |
| 926 | but not using rsh/rlogin. |
Damien Miller | f1d9a30 | 1999-10-29 09:17:36 +1000 | [diff] [blame] | 927 | .It Pa /etc/ssh/sshrc |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 928 | Commands in this file are executed by |
| 929 | .Nm |
| 930 | when the user logs in just before the user's shell (or command) is started. |
| 931 | See the |
| 932 | .Xr sshd 8 |
| 933 | manual page for more information. |
| 934 | .It Pa $HOME/.ssh/rc |
| 935 | Commands in this file are executed by |
| 936 | .Nm |
| 937 | when the user logs in just before the user's shell (or command) is |
| 938 | started. |
| 939 | See the |
| 940 | .Xr sshd 8 |
| 941 | manual page for more information. |
| 942 | .It Pa libcrypto.so.X.1 |
| 943 | A version of this library which includes support for the RSA algorithm |
| 944 | is required for proper operation. |
| 945 | .Sh AUTHOR |
| 946 | Tatu Ylonen <ylo@cs.hut.fi> |
| 947 | .Pp |
| 948 | Issues can be found from the SSH WWW home page: |
| 949 | .Pp |
| 950 | .Dl http://www.cs.hut.fi/ssh |
| 951 | .Pp |
| 952 | OpenSSH |
| 953 | is a derivative of the original (free) ssh 1.2.12 release, but with bugs |
| 954 | removed and newer features re-added. Rapidly after the 1.2.12 release, |
| 955 | newer versions bore successively more restrictive licenses. This version |
| 956 | of OpenSSH |
| 957 | .Bl -bullet |
| 958 | .It |
| 959 | has all components of a restrictive nature (ie. patents, see |
| 960 | .Xr ssl 8 ) |
| 961 | directly removed from the source code; any licensed or patented components |
| 962 | are chosen from |
| 963 | external libraries. |
| 964 | .It |
| 965 | has been updated to support ssh protocol 1.5. |
| 966 | .It |
| 967 | contains added support for |
| 968 | .Xr kerberos 8 |
| 969 | authentication and ticket passing. |
| 970 | .It |
| 971 | supports one-time password authentication with |
| 972 | .Xr skey 1 . |
| 973 | .El |
| 974 | .Pp |
| 975 | The libraries described in |
| 976 | .Xr ssl 8 |
| 977 | are required for proper operation. |
Damien Miller | 10f6f6b | 1999-11-17 17:29:08 +1100 | [diff] [blame] | 978 | .Pp |
Damien Miller | 6ee9564 | 1999-11-18 11:35:13 +1100 | [diff] [blame] | 979 | OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl, |
Damien Miller | 10f6f6b | 1999-11-17 17:29:08 +1100 | [diff] [blame] | 980 | Niels Provos, Theo de Raadt, and Dug Song. |
Damien Miller | 32aa144 | 1999-10-29 09:15:49 +1000 | [diff] [blame] | 981 | .Sh SEE ALSO |
| 982 | .Xr rlogin 1 , |
| 983 | .Xr rsh 1 , |
| 984 | .Xr scp 1 , |
| 985 | .Xr ssh-add 1 , |
| 986 | .Xr ssh-agent 1 , |
| 987 | .Xr ssh-keygen 1 , |
| 988 | .Xr telnet 1 , |
| 989 | .Xr sshd 8 , |
| 990 | .Xr ssl 8 |