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.\" -*- nroff -*- |
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.\" |
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.\" Author: Tatu Ylonen <ylo@cs.hut.fi> |
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.\" Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland |
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.\" All rights reserved |
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.\" |
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.\" As far as I am concerned, the code I have written for this software |
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.\" can be used freely for any purpose. Any derived versions of this |
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.\" software must be clearly marked as such, and if the derived work is |
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.\" incompatible with the protocol description in the RFC file, it must be |
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.\" called by a name other than "ssh" or "Secure Shell". |
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.\" |
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.\" Copyright (c) 1999,2000 Markus Friedl. All rights reserved. |
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.\" Copyright (c) 1999 Aaron Campbell. All rights reserved. |
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.\" Copyright (c) 1999 Theo de Raadt. All rights reserved. |
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.\" |
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.\" Redistribution and use in source and binary forms, with or without |
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.\" modification, are permitted provided that the following conditions |
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.\" are met: |
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.\" 1. Redistributions of source code must retain the above copyright |
| 21 |
.\" notice, this list of conditions and the following disclaimer. |
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.\" 2. Redistributions in binary form must reproduce the above copyright |
| 23 |
.\" notice, this list of conditions and the following disclaimer in the |
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.\" documentation and/or other materials provided with the distribution. |
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.\" |
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR |
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.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES |
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.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. |
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.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, |
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.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT |
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.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, |
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.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
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.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
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.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF |
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.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
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.\" |
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.\" $OpenBSD: ssh.1,v 1.253 2006/01/30 13:37:49 jmc Exp $ |
| 38 |
.Dd September 25, 1999 |
| 39 |
.Dt SSH 1 |
| 40 |
.Os |
| 41 |
.Sh NAME |
| 42 |
.Nm ssh |
| 43 |
.Nd OpenSSH SSH client (remote login program) |
| 44 |
.Sh SYNOPSIS |
| 45 |
.Nm ssh |
| 46 |
.Op Fl 1246AaCfgKkMNnqsTtVvXxY |
| 47 |
.Op Fl b Ar bind_address |
| 48 |
.Op Fl c Ar cipher_spec |
| 49 |
.Oo Fl D\ \& |
| 50 |
.Sm off |
| 51 |
.Oo Ar bind_address : Oc |
| 52 |
.Ar port |
| 53 |
.Sm on |
| 54 |
.Oc |
| 55 |
.Op Fl e Ar escape_char |
| 56 |
.Op Fl F Ar configfile |
| 57 |
.Bk -words |
| 58 |
.Op Fl i Ar identity_file |
| 59 |
.Ek |
| 60 |
.Oo Fl L\ \& |
| 61 |
.Sm off |
| 62 |
.Oo Ar bind_address : Oc |
| 63 |
.Ar port : host : hostport |
| 64 |
.Sm on |
| 65 |
.Oc |
| 66 |
.Bk -words |
| 67 |
.Op Fl l Ar login_name |
| 68 |
.Ek |
| 69 |
.Op Fl m Ar mac_spec |
| 70 |
.Op Fl O Ar ctl_cmd |
| 71 |
.Op Fl o Ar option |
| 72 |
.Op Fl p Ar port |
| 73 |
.Oo Fl R\ \& |
| 74 |
.Sm off |
| 75 |
.Oo Ar bind_address : Oc |
| 76 |
.Ar port : host : hostport |
| 77 |
.Sm on |
| 78 |
.Oc |
| 79 |
.Op Fl S Ar ctl_path |
| 80 |
.Bk -words |
| 81 |
.Op Fl w Ar tunnel : Ns Ar tunnel |
| 82 |
.Oo Ar user Ns @ Oc Ns Ar hostname |
| 83 |
.Op Ar command |
| 84 |
.Ek |
| 85 |
.Sh DESCRIPTION |
| 86 |
.Nm |
| 87 |
(SSH client) is a program for logging into a remote machine and for |
| 88 |
executing commands on a remote machine. |
| 89 |
It is intended to replace rlogin and rsh, |
| 90 |
and provide secure encrypted communications between |
| 91 |
two untrusted hosts over an insecure network. |
| 92 |
X11 connections and arbitrary TCP ports |
| 93 |
can also be forwarded over the secure channel. |
| 94 |
.Pp |
| 95 |
.Nm |
| 96 |
connects and logs into the specified |
| 97 |
.Ar hostname |
| 98 |
(with optional |
| 99 |
.Ar user |
| 100 |
name). |
| 101 |
The user must prove |
| 102 |
his/her identity to the remote machine using one of several methods |
| 103 |
depending on the protocol version used (see below). |
| 104 |
.Pp |
| 105 |
If |
| 106 |
.Ar command |
| 107 |
is specified, |
| 108 |
it is executed on the remote host instead of a login shell. |
| 109 |
.Pp |
| 110 |
The options are as follows: |
| 111 |
.Bl -tag -width Ds |
| 112 |
.It Fl 1 |
| 113 |
Forces |
| 114 |
.Nm |
| 115 |
to try protocol version 1 only. |
| 116 |
.It Fl 2 |
| 117 |
Forces |
| 118 |
.Nm |
| 119 |
to try protocol version 2 only. |
| 120 |
.It Fl 4 |
| 121 |
Forces |
| 122 |
.Nm |
| 123 |
to use IPv4 addresses only. |
| 124 |
.It Fl 6 |
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Forces |
| 126 |
.Nm |
| 127 |
to use IPv6 addresses only. |
| 128 |
.It Fl A |
| 129 |
Enables forwarding of the authentication agent connection. |
| 130 |
This can also be specified on a per-host basis in a configuration file. |
| 131 |
.Pp |
| 132 |
Agent forwarding should be enabled with caution. |
| 133 |
Users with the ability to bypass file permissions on the remote host |
| 134 |
(for the agent's Unix-domain socket) |
| 135 |
can access the local agent through the forwarded connection. |
| 136 |
An attacker cannot obtain key material from the agent, |
| 137 |
however they can perform operations on the keys that enable them to |
| 138 |
authenticate using the identities loaded into the agent. |
| 139 |
.It Fl a |
| 140 |
Disables forwarding of the authentication agent connection. |
| 141 |
.It Fl b Ar bind_address |
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Use |
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.Ar bind_address |
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on the local machine as the source address |
| 145 |
of the connection. |
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Only useful on systems with more than one address. |
| 147 |
.It Fl C |
| 148 |
Requests compression of all data (including stdin, stdout, stderr, and |
| 149 |
data for forwarded X11 and TCP connections). |
| 150 |
The compression algorithm is the same used by |
| 151 |
.Xr gzip 1 , |
| 152 |
and the |
| 153 |
.Dq level |
| 154 |
can be controlled by the |
| 155 |
.Cm CompressionLevel |
| 156 |
option for protocol version 1. |
| 157 |
Compression is desirable on modem lines and other |
| 158 |
slow connections, but will only slow down things on fast networks. |
| 159 |
The default value can be set on a host-by-host basis in the |
| 160 |
configuration files; see the |
| 161 |
.Cm Compression |
| 162 |
option. |
| 163 |
.It Fl c Ar cipher_spec |
| 164 |
Selects the cipher specification for encrypting the session. |
| 165 |
.Pp |
| 166 |
Protocol version 1 allows specification of a single cipher. |
| 167 |
The supported values are |
| 168 |
.Dq 3des , |
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.Dq blowfish , |
| 170 |
and |
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.Dq des . |
| 172 |
.Ar 3des |
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(triple-des) is an encrypt-decrypt-encrypt triple with three different keys. |
| 174 |
It is believed to be secure. |
| 175 |
.Ar blowfish |
| 176 |
is a fast block cipher; it appears very secure and is much faster than |
| 177 |
.Ar 3des . |
| 178 |
.Ar des |
| 179 |
is only supported in the |
| 180 |
.Nm |
| 181 |
client for interoperability with legacy protocol 1 implementations |
| 182 |
that do not support the |
| 183 |
.Ar 3des |
| 184 |
cipher. |
| 185 |
Its use is strongly discouraged due to cryptographic weaknesses. |
| 186 |
The default is |
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.Dq 3des . |
| 188 |
.Pp |
| 189 |
For protocol version 2, |
| 190 |
.Ar cipher_spec |
| 191 |
is a comma-separated list of ciphers |
| 192 |
listed in order of preference. |
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The supported ciphers are: |
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3des-cbc, |
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aes128-cbc, |
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aes192-cbc, |
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aes256-cbc, |
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aes128-ctr, |
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aes192-ctr, |
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aes256-ctr, |
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arcfour128, |
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arcfour256, |
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arcfour, |
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blowfish-cbc, |
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and |
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cast128-cbc. |
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The default is: |
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.Bd -literal -offset indent |
| 209 |
aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, |
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arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, |
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aes192-ctr,aes256-ctr |
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.Ed |
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.It Fl D Xo |
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.Sm off |
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.Oo Ar bind_address : Oc |
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.Ar port |
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.Sm on |
| 218 |
.Xc |
| 219 |
Specifies a local |
| 220 |
.Dq dynamic |
| 221 |
application-level port forwarding. |
| 222 |
This works by allocating a socket to listen to |
| 223 |
.Ar port |
| 224 |
on the local side, optionally bound to the specified |
| 225 |
.Ar bind_address . |
| 226 |
Whenever a connection is made to this port, the |
| 227 |
connection is forwarded over the secure channel, and the application |
| 228 |
protocol is then used to determine where to connect to from the |
| 229 |
remote machine. |
| 230 |
Currently the SOCKS4 and SOCKS5 protocols are supported, and |
| 231 |
.Nm |
| 232 |
will act as a SOCKS server. |
| 233 |
Only root can forward privileged ports. |
| 234 |
Dynamic port forwardings can also be specified in the configuration file. |
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.Pp |
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IPv6 addresses can be specified with an alternative syntax: |
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.Sm off |
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.Xo |
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.Op Ar bind_address No / |
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.Ar port |
| 241 |
.Xc |
| 242 |
.Sm on |
| 243 |
or by enclosing the address in square brackets. |
| 244 |
Only the superuser can forward privileged ports. |
| 245 |
By default, the local port is bound in accordance with the |
| 246 |
.Cm GatewayPorts |
| 247 |
setting. |
| 248 |
However, an explicit |
| 249 |
.Ar bind_address |
| 250 |
may be used to bind the connection to a specific address. |
| 251 |
The |
| 252 |
.Ar bind_address |
| 253 |
of |
| 254 |
.Dq localhost |
| 255 |
indicates that the listening port be bound for local use only, while an |
| 256 |
empty address or |
| 257 |
.Sq * |
| 258 |
indicates that the port should be available from all interfaces. |
| 259 |
.It Fl e Ar escape_char |
| 260 |
Sets the escape character for sessions with a pty (default: |
| 261 |
.Ql ~ ) . |
| 262 |
The escape character is only recognized at the beginning of a line. |
| 263 |
The escape character followed by a dot |
| 264 |
.Pq Ql \&. |
| 265 |
closes the connection; |
| 266 |
followed by control-Z suspends the connection; |
| 267 |
and followed by itself sends the escape character once. |
| 268 |
Setting the character to |
| 269 |
.Dq none |
| 270 |
disables any escapes and makes the session fully transparent. |
| 271 |
.It Fl F Ar configfile |
| 272 |
Specifies an alternative per-user configuration file. |
| 273 |
If a configuration file is given on the command line, |
| 274 |
the system-wide configuration file |
| 275 |
.Pq Pa /usr/etc/ssh_config |
| 276 |
will be ignored. |
| 277 |
The default for the per-user configuration file is |
| 278 |
.Pa ~/.ssh/config . |
| 279 |
.It Fl f |
| 280 |
Requests |
| 281 |
.Nm |
| 282 |
to go to background just before command execution. |
| 283 |
This is useful if |
| 284 |
.Nm |
| 285 |
is going to ask for passwords or passphrases, but the user |
| 286 |
wants it in the background. |
| 287 |
This implies |
| 288 |
.Fl n . |
| 289 |
The recommended way to start X11 programs at a remote site is with |
| 290 |
something like |
| 291 |
.Ic ssh -f host xterm . |
| 292 |
.It Fl g |
| 293 |
Allows remote hosts to connect to local forwarded ports. |
| 294 |
.It Fl I Ar smartcard_device |
| 295 |
Specify the device |
| 296 |
.Nm |
| 297 |
should use to communicate with a smartcard used for storing the user's |
| 298 |
private RSA key. |
| 299 |
This option is only available if support for smartcard devices |
| 300 |
is compiled in (default is no support). |
| 301 |
.It Fl i Ar identity_file |
| 302 |
Selects a file from which the identity (private key) for |
| 303 |
RSA or DSA authentication is read. |
| 304 |
The default is |
| 305 |
.Pa ~/.ssh/identity |
| 306 |
for protocol version 1, and |
| 307 |
.Pa ~/.ssh/id_rsa |
| 308 |
and |
| 309 |
.Pa ~/.ssh/id_dsa |
| 310 |
for protocol version 2. |
| 311 |
Identity files may also be specified on |
| 312 |
a per-host basis in the configuration file. |
| 313 |
It is possible to have multiple |
| 314 |
.Fl i |
| 315 |
options (and multiple identities specified in |
| 316 |
configuration files). |
| 317 |
.It Fl K |
| 318 |
Enables forwarding (delegation) of GSSAPI credentials to the server. |
| 319 |
.It Fl k |
| 320 |
Disables forwarding (delegation) of GSSAPI credentials to the server. |
| 321 |
.It Fl L Xo |
| 322 |
.Sm off |
| 323 |
.Oo Ar bind_address : Oc |
| 324 |
.Ar port : host : hostport |
| 325 |
.Sm on |
| 326 |
.Xc |
| 327 |
Specifies that the given port on the local (client) host is to be |
| 328 |
forwarded to the given host and port on the remote side. |
| 329 |
This works by allocating a socket to listen to |
| 330 |
.Ar port |
| 331 |
on the local side, optionally bound to the specified |
| 332 |
.Ar bind_address . |
| 333 |
Whenever a connection is made to this port, the |
| 334 |
connection is forwarded over the secure channel, and a connection is |
| 335 |
made to |
| 336 |
.Ar host |
| 337 |
port |
| 338 |
.Ar hostport |
| 339 |
from the remote machine. |
| 340 |
Port forwardings can also be specified in the configuration file. |
| 341 |
IPv6 addresses can be specified with an alternative syntax: |
| 342 |
.Sm off |
| 343 |
.Xo |
| 344 |
.Op Ar bind_address No / |
| 345 |
.Ar port No / Ar host No / |
| 346 |
.Ar hostport |
| 347 |
.Xc |
| 348 |
.Sm on |
| 349 |
or by enclosing the address in square brackets. |
| 350 |
Only the superuser can forward privileged ports. |
| 351 |
By default, the local port is bound in accordance with the |
| 352 |
.Cm GatewayPorts |
| 353 |
setting. |
| 354 |
However, an explicit |
| 355 |
.Ar bind_address |
| 356 |
may be used to bind the connection to a specific address. |
| 357 |
The |
| 358 |
.Ar bind_address |
| 359 |
of |
| 360 |
.Dq localhost |
| 361 |
indicates that the listening port be bound for local use only, while an |
| 362 |
empty address or |
| 363 |
.Sq * |
| 364 |
indicates that the port should be available from all interfaces. |
| 365 |
.It Fl l Ar login_name |
| 366 |
Specifies the user to log in as on the remote machine. |
| 367 |
This also may be specified on a per-host basis in the configuration file. |
| 368 |
.It Fl M |
| 369 |
Places the |
| 370 |
.Nm |
| 371 |
client into |
| 372 |
.Dq master |
| 373 |
mode for connection sharing. |
| 374 |
Multiple |
| 375 |
.Fl M |
| 376 |
options places |
| 377 |
.Nm |
| 378 |
into |
| 379 |
.Dq master |
| 380 |
mode with confirmation required before slave connections are accepted. |
| 381 |
Refer to the description of |
| 382 |
.Cm ControlMaster |
| 383 |
in |
| 384 |
.Xr ssh_config 5 |
| 385 |
for details. |
| 386 |
.It Fl m Ar mac_spec |
| 387 |
Additionally, for protocol version 2 a comma-separated list of MAC |
| 388 |
(message authentication code) algorithms can |
| 389 |
be specified in order of preference. |
| 390 |
See the |
| 391 |
.Cm MACs |
| 392 |
keyword for more information. |
| 393 |
.It Fl N |
| 394 |
Do not execute a remote command. |
| 395 |
This is useful for just forwarding ports |
| 396 |
(protocol version 2 only). |
| 397 |
.It Fl n |
| 398 |
Redirects stdin from |
| 399 |
.Pa /dev/null |
| 400 |
(actually, prevents reading from stdin). |
| 401 |
This must be used when |
| 402 |
.Nm |
| 403 |
is run in the background. |
| 404 |
A common trick is to use this to run X11 programs on a remote machine. |
| 405 |
For example, |
| 406 |
.Ic ssh -n shadows.cs.hut.fi emacs & |
| 407 |
will start an emacs on shadows.cs.hut.fi, and the X11 |
| 408 |
connection will be automatically forwarded over an encrypted channel. |
| 409 |
The |
| 410 |
.Nm |
| 411 |
program will be put in the background. |
| 412 |
(This does not work if |
| 413 |
.Nm |
| 414 |
needs to ask for a password or passphrase; see also the |
| 415 |
.Fl f |
| 416 |
option.) |
| 417 |
.It Fl O Ar ctl_cmd |
| 418 |
Control an active connection multiplexing master process. |
| 419 |
When the |
| 420 |
.Fl O |
| 421 |
option is specified, the |
| 422 |
.Ar ctl_cmd |
| 423 |
argument is interpreted and passed to the master process. |
| 424 |
Valid commands are: |
| 425 |
.Dq check |
| 426 |
(check that the master process is running) and |
| 427 |
.Dq exit |
| 428 |
(request the master to exit). |
| 429 |
.It Fl o Ar option |
| 430 |
Can be used to give options in the format used in the configuration file. |
| 431 |
This is useful for specifying options for which there is no separate |
| 432 |
command-line flag. |
| 433 |
For full details of the options listed below, and their possible values, see |
| 434 |
.Xr ssh_config 5 . |
| 435 |
.Pp |
| 436 |
.Bl -tag -width Ds -offset indent -compact |
| 437 |
.It AddressFamily |
| 438 |
.It BatchMode |
| 439 |
.It BindAddress |
| 440 |
.It ChallengeResponseAuthentication |
| 441 |
.It CheckHostIP |
| 442 |
.It Cipher |
| 443 |
.It Ciphers |
| 444 |
.It ClearAllForwardings |
| 445 |
.It Compression |
| 446 |
.It CompressionLevel |
| 447 |
.It ConnectionAttempts |
| 448 |
.It ConnectTimeout |
| 449 |
.It ControlMaster |
| 450 |
.It ControlPath |
| 451 |
.It DynamicForward |
| 452 |
.It EscapeChar |
| 453 |
.It ForwardAgent |
| 454 |
.It ForwardX11 |
| 455 |
.It ForwardX11Trusted |
| 456 |
.It GatewayPorts |
| 457 |
.It GlobalKnownHostsFile |
| 458 |
.It GSSAPIAuthentication |
| 459 |
.It GSSAPIDelegateCredentials |
| 460 |
.It HashKnownHosts |
| 461 |
.It Host |
| 462 |
.It HostbasedAuthentication |
| 463 |
.It HostKeyAlgorithms |
| 464 |
.It HostKeyAlias |
| 465 |
.It HostName |
| 466 |
.It IdentityFile |
| 467 |
.It IdentitiesOnly |
| 468 |
.It KbdInteractiveDevices |
| 469 |
.It LocalCommand |
| 470 |
.It LocalForward |
| 471 |
.It LogLevel |
| 472 |
.It MACs |
| 473 |
.It NoHostAuthenticationForLocalhost |
| 474 |
.It NumberOfPasswordPrompts |
| 475 |
.It PasswordAuthentication |
| 476 |
.It PermitLocalCommand |
| 477 |
.It Port |
| 478 |
.It PreferredAuthentications |
| 479 |
.It Protocol |
| 480 |
.It ProxyCommand |
| 481 |
.It PubkeyAuthentication |
| 482 |
.It RekeyLimit |
| 483 |
.It RemoteForward |
| 484 |
.It RhostsRSAAuthentication |
| 485 |
.It RSAAuthentication |
| 486 |
.It SendEnv |
| 487 |
.It ServerAliveInterval |
| 488 |
.It ServerAliveCountMax |
| 489 |
.It SmartcardDevice |
| 490 |
.It StrictHostKeyChecking |
| 491 |
.It TCPKeepAlive |
| 492 |
.It Tunnel |
| 493 |
.It TunnelDevice |
| 494 |
.It UsePrivilegedPort |
| 495 |
.It User |
| 496 |
.It UserKnownHostsFile |
| 497 |
.It VerifyHostKeyDNS |
| 498 |
.It XAuthLocation |
| 499 |
.El |
| 500 |
.It Fl p Ar port |
| 501 |
Port to connect to on the remote host. |
| 502 |
This can be specified on a |
| 503 |
per-host basis in the configuration file. |
| 504 |
.It Fl q |
| 505 |
Quiet mode. |
| 506 |
Causes all warning and diagnostic messages to be suppressed. |
| 507 |
Only fatal errors are displayed. |
| 508 |
If a second |
| 509 |
.Fl q |
| 510 |
is given then even fatal errors are suppressed. |
| 511 |
.It Fl R Xo |
| 512 |
.Sm off |
| 513 |
.Oo Ar bind_address : Oc |
| 514 |
.Ar port : host : hostport |
| 515 |
.Sm on |
| 516 |
.Xc |
| 517 |
Specifies that the given port on the remote (server) host is to be |
| 518 |
forwarded to the given host and port on the local side. |
| 519 |
This works by allocating a socket to listen to |
| 520 |
.Ar port |
| 521 |
on the remote side, and whenever a connection is made to this port, the |
| 522 |
connection is forwarded over the secure channel, and a connection is |
| 523 |
made to |
| 524 |
.Ar host |
| 525 |
port |
| 526 |
.Ar hostport |
| 527 |
from the local machine. |
| 528 |
.Pp |
| 529 |
Port forwardings can also be specified in the configuration file. |
| 530 |
Privileged ports can be forwarded only when |
| 531 |
logging in as root on the remote machine. |
| 532 |
IPv6 addresses can be specified by enclosing the address in square braces or |
| 533 |
using an alternative syntax: |
| 534 |
.Sm off |
| 535 |
.Xo |
| 536 |
.Op Ar bind_address No / |
| 537 |
.Ar host No / Ar port No / |
| 538 |
.Ar hostport |
| 539 |
.Xc . |
| 540 |
.Sm on |
| 541 |
.Pp |
| 542 |
By default, the listening socket on the server will be bound to the loopback |
| 543 |
interface only. |
| 544 |
This may be overriden by specifying a |
| 545 |
.Ar bind_address . |
| 546 |
An empty |
| 547 |
.Ar bind_address , |
| 548 |
or the address |
| 549 |
.Ql * , |
| 550 |
indicates that the remote socket should listen on all interfaces. |
| 551 |
Specifying a remote |
| 552 |
.Ar bind_address |
| 553 |
will only succeed if the server's |
| 554 |
.Cm GatewayPorts |
| 555 |
option is enabled (see |
| 556 |
.Xr sshd_config 5 ) . |
| 557 |
.It Fl S Ar ctl_path |
| 558 |
Specifies the location of a control socket for connection sharing. |
| 559 |
Refer to the description of |
| 560 |
.Cm ControlPath |
| 561 |
and |
| 562 |
.Cm ControlMaster |
| 563 |
in |
| 564 |
.Xr ssh_config 5 |
| 565 |
for details. |
| 566 |
.It Fl s |
| 567 |
May be used to request invocation of a subsystem on the remote system. |
| 568 |
Subsystems are a feature of the SSH2 protocol which facilitate the use |
| 569 |
of SSH as a secure transport for other applications (eg.\& |
| 570 |
.Xr sftp 1 ) . |
| 571 |
The subsystem is specified as the remote command. |
| 572 |
.It Fl T |
| 573 |
Disable pseudo-tty allocation. |
| 574 |
.It Fl t |
| 575 |
Force pseudo-tty allocation. |
| 576 |
This can be used to execute arbitrary |
| 577 |
screen-based programs on a remote machine, which can be very useful, |
| 578 |
e.g., when implementing menu services. |
| 579 |
Multiple |
| 580 |
.Fl t |
| 581 |
options force tty allocation, even if |
| 582 |
.Nm |
| 583 |
has no local tty. |
| 584 |
.It Fl V |
| 585 |
Display the version number and exit. |
| 586 |
.It Fl v |
| 587 |
Verbose mode. |
| 588 |
Causes |
| 589 |
.Nm |
| 590 |
to print debugging messages about its progress. |
| 591 |
This is helpful in |
| 592 |
debugging connection, authentication, and configuration problems. |
| 593 |
Multiple |
| 594 |
.Fl v |
| 595 |
options increase the verbosity. |
| 596 |
The maximum is 3. |
| 597 |
.It Fl w Ar tunnel : Ns Ar tunnel |
| 598 |
Requests a |
| 599 |
.Xr tun 4 |
| 600 |
device on the client |
| 601 |
(first |
| 602 |
.Ar tunnel |
| 603 |
arg) |
| 604 |
and server |
| 605 |
(second |
| 606 |
.Ar tunnel |
| 607 |
arg). |
| 608 |
The devices may be specified by numerical ID or the keyword |
| 609 |
.Dq any , |
| 610 |
which uses the next available tunnel device. |
| 611 |
See also the |
| 612 |
.Cm Tunnel |
| 613 |
directive in |
| 614 |
.Xr ssh_config 5 . |
| 615 |
.It Fl X |
| 616 |
Enables X11 forwarding. |
| 617 |
This can also be specified on a per-host basis in a configuration file. |
| 618 |
.Pp |
| 619 |
X11 forwarding should be enabled with caution. |
| 620 |
Users with the ability to bypass file permissions on the remote host |
| 621 |
(for the user's X authorization database) |
| 622 |
can access the local X11 display through the forwarded connection. |
| 623 |
An attacker may then be able to perform activities such as keystroke monitoring. |
| 624 |
.Pp |
| 625 |
For this reason, X11 forwarding is subjected to X11 SECURITY extension |
| 626 |
restrictions by default. |
| 627 |
Please refer to the |
| 628 |
.Nm |
| 629 |
.Fl Y |
| 630 |
option and the |
| 631 |
.Cm ForwardX11Trusted |
| 632 |
directive in |
| 633 |
.Xr ssh_config 5 |
| 634 |
for more information. |
| 635 |
.It Fl x |
| 636 |
Disables X11 forwarding. |
| 637 |
.It Fl Y |
| 638 |
Enables trusted X11 forwarding. |
| 639 |
Trusted X11 forwardings are not subjected to the X11 SECURITY extension |
| 640 |
controls. |
| 641 |
.El |
| 642 |
.Pp |
| 643 |
.Nm |
| 644 |
may additionally obtain configuration data from |
| 645 |
a per-user configuration file and a system-wide configuration file. |
| 646 |
The file format and configuration options are described in |
| 647 |
.Xr ssh_config 5 . |
| 648 |
.Pp |
| 649 |
.Nm |
| 650 |
exits with the exit status of the remote command or with 255 |
| 651 |
if an error occurred. |
| 652 |
.Sh AUTHENTICATION |
| 653 |
The OpenSSH SSH client supports SSH protocols 1 and 2. |
| 654 |
Protocol 2 is the default, with |
| 655 |
.Nm |
| 656 |
falling back to protocol 1 if it detects protocol 2 is unsupported. |
| 657 |
These settings may be altered using the |
| 658 |
.Cm Protocol |
| 659 |
option in |
| 660 |
.Xr ssh_config 5 , |
| 661 |
or enforced using the |
| 662 |
.Fl 1 |
| 663 |
and |
| 664 |
.Fl 2 |
| 665 |
options (see above). |
| 666 |
Both protocols support similar authentication methods, |
| 667 |
but protocol 2 is preferred since |
| 668 |
it provides additional mechanisms for confidentiality |
| 669 |
(the traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) |
| 670 |
and integrity (hmac-md5, hmac-sha1, hmac-ripemd160). |
| 671 |
Protocol 1 lacks a strong mechanism for ensuring the |
| 672 |
integrity of the connection. |
| 673 |
.Pp |
| 674 |
The methods available for authentication are: |
| 675 |
host-based authentication, |
| 676 |
public key authentication, |
| 677 |
challenge-response authentication, |
| 678 |
and password authentication. |
| 679 |
Authentication methods are tried in the order specified above, |
| 680 |
though protocol 2 has a configuration option to change the default order: |
| 681 |
.Cm PreferredAuthentications . |
| 682 |
.Pp |
| 683 |
Host-based authentication works as follows: |
| 684 |
If the machine the user logs in from is listed in |
| 685 |
.Pa /etc/hosts.equiv |
| 686 |
or |
| 687 |
.Pa /usr/etc/shosts.equiv |
| 688 |
on the remote machine, and the user names are |
| 689 |
the same on both sides, or if the files |
| 690 |
.Pa ~/.rhosts |
| 691 |
or |
| 692 |
.Pa ~/.shosts |
| 693 |
exist in the user's home directory on the |
| 694 |
remote machine and contain a line containing the name of the client |
| 695 |
machine and the name of the user on that machine, the user is |
| 696 |
considered for login. |
| 697 |
Additionally, the server |
| 698 |
.Em must |
| 699 |
be able to verify the client's |
| 700 |
host key (see the description of |
| 701 |
.Pa /usr/etc/ssh_known_hosts |
| 702 |
and |
| 703 |
.Pa ~/.ssh/known_hosts , |
| 704 |
below) |
| 705 |
for login to be permitted. |
| 706 |
This authentication method closes security holes due to IP |
| 707 |
spoofing, DNS spoofing, and routing spoofing. |
| 708 |
[Note to the administrator: |
| 709 |
.Pa /etc/hosts.equiv , |
| 710 |
.Pa ~/.rhosts , |
| 711 |
and the rlogin/rsh protocol in general, are inherently insecure and should be |
| 712 |
disabled if security is desired.] |
| 713 |
.Pp |
| 714 |
Public key authentication works as follows: |
| 715 |
The scheme is based on public-key cryptography, |
| 716 |
using cryptosystems |
| 717 |
where encryption and decryption are done using separate keys, |
| 718 |
and it is unfeasible to derive the decryption key from the encryption key. |
| 719 |
The idea is that each user creates a public/private |
| 720 |
key pair for authentication purposes. |
| 721 |
The server knows the public key, and only the user knows the private key. |
| 722 |
.Nm |
| 723 |
implements public key authentication protocol automatically, |
| 724 |
using either the RSA or DSA algorithms. |
| 725 |
Protocol 1 is restricted to using only RSA keys, |
| 726 |
but protocol 2 may use either. |
| 727 |
The |
| 728 |
.Sx HISTORY |
| 729 |
section of |
| 730 |
.Xr ssl 8 |
| 731 |
contains a brief discussion of the two algorithms. |
| 732 |
.Pp |
| 733 |
The file |
| 734 |
.Pa ~/.ssh/authorized_keys |
| 735 |
lists the public keys that are permitted for logging in. |
| 736 |
When the user logs in, the |
| 737 |
.Nm |
| 738 |
program tells the server which key pair it would like to use for |
| 739 |
authentication. |
| 740 |
The client proves that it has access to the private key |
| 741 |
and the server checks that the corresponding public key |
| 742 |
is authorized to accept the account. |
| 743 |
.Pp |
| 744 |
The user creates his/her key pair by running |
| 745 |
.Xr ssh-keygen 1 . |
| 746 |
This stores the private key in |
| 747 |
.Pa ~/.ssh/identity |
| 748 |
(protocol 1), |
| 749 |
.Pa ~/.ssh/id_dsa |
| 750 |
(protocol 2 DSA), |
| 751 |
or |
| 752 |
.Pa ~/.ssh/id_rsa |
| 753 |
(protocol 2 RSA) |
| 754 |
and stores the public key in |
| 755 |
.Pa ~/.ssh/identity.pub |
| 756 |
(protocol 1), |
| 757 |
.Pa ~/.ssh/id_dsa.pub |
| 758 |
(protocol 2 DSA), |
| 759 |
or |
| 760 |
.Pa ~/.ssh/id_rsa.pub |
| 761 |
(protocol 2 RSA) |
| 762 |
in the user's home directory. |
| 763 |
The user should then copy the public key |
| 764 |
to |
| 765 |
.Pa ~/.ssh/authorized_keys |
| 766 |
in his/her home directory on the remote machine. |
| 767 |
The |
| 768 |
.Pa authorized_keys |
| 769 |
file corresponds to the conventional |
| 770 |
.Pa ~/.rhosts |
| 771 |
file, and has one key |
| 772 |
per line, though the lines can be very long. |
| 773 |
After this, the user can log in without giving the password. |
| 774 |
.Pp |
| 775 |
The most convenient way to use public key authentication may be with an |
| 776 |
authentication agent. |
| 777 |
See |
| 778 |
.Xr ssh-agent 1 |
| 779 |
for more information. |
| 780 |
.Pp |
| 781 |
Challenge-response authentication works as follows: |
| 782 |
The server sends an arbitrary |
| 783 |
.Qq challenge |
| 784 |
text, and prompts for a response. |
| 785 |
Protocol 2 allows multiple challenges and responses; |
| 786 |
protocol 1 is restricted to just one challenge/response. |
| 787 |
Examples of challenge-response authentication include |
| 788 |
BSD Authentication (see |
| 789 |
.Xr login.conf 5 ) |
| 790 |
and PAM (some non-OpenBSD systems). |
| 791 |
.Pp |
| 792 |
Finally, if other authentication methods fail, |
| 793 |
.Nm |
| 794 |
prompts the user for a password. |
| 795 |
The password is sent to the remote |
| 796 |
host for checking; however, since all communications are encrypted, |
| 797 |
the password cannot be seen by someone listening on the network. |
| 798 |
.Pp |
| 799 |
.Nm |
| 800 |
automatically maintains and checks a database containing |
| 801 |
identification for all hosts it has ever been used with. |
| 802 |
Host keys are stored in |
| 803 |
.Pa ~/.ssh/known_hosts |
| 804 |
in the user's home directory. |
| 805 |
Additionally, the file |
| 806 |
.Pa /usr/etc/ssh_known_hosts |
| 807 |
is automatically checked for known hosts. |
| 808 |
Any new hosts are automatically added to the user's file. |
| 809 |
If a host's identification ever changes, |
| 810 |
.Nm |
| 811 |
warns about this and disables password authentication to prevent |
| 812 |
server spoofing or man-in-the-middle attacks, |
| 813 |
which could otherwise be used to circumvent the encryption. |
| 814 |
The |
| 815 |
.Cm StrictHostKeyChecking |
| 816 |
option can be used to control logins to machines whose |
| 817 |
host key is not known or has changed. |
| 818 |
.Pp |
| 819 |
When the user's identity has been accepted by the server, the server |
| 820 |
either executes the given command, or logs into the machine and gives |
| 821 |
the user a normal shell on the remote machine. |
| 822 |
All communication with |
| 823 |
the remote command or shell will be automatically encrypted. |
| 824 |
.Pp |
| 825 |
If a pseudo-terminal has been allocated (normal login session), the |
| 826 |
user may use the escape characters noted below. |
| 827 |
.Pp |
| 828 |
If no pseudo-tty has been allocated, |
| 829 |
the session is transparent and can be used to reliably transfer binary data. |
| 830 |
On most systems, setting the escape character to |
| 831 |
.Dq none |
| 832 |
will also make the session transparent even if a tty is used. |
| 833 |
.Pp |
| 834 |
The session terminates when the command or shell on the remote |
| 835 |
machine exits and all X11 and TCP connections have been closed. |
| 836 |
.Sh ESCAPE CHARACTERS |
| 837 |
When a pseudo-terminal has been requested, |
| 838 |
.Nm |
| 839 |
supports a number of functions through the use of an escape character. |
| 840 |
.Pp |
| 841 |
A single tilde character can be sent as |
| 842 |
.Ic ~~ |
| 843 |
or by following the tilde by a character other than those described below. |
| 844 |
The escape character must always follow a newline to be interpreted as |
| 845 |
special. |
| 846 |
The escape character can be changed in configuration files using the |
| 847 |
.Cm EscapeChar |
| 848 |
configuration directive or on the command line by the |
| 849 |
.Fl e |
| 850 |
option. |
| 851 |
.Pp |
| 852 |
The supported escapes (assuming the default |
| 853 |
.Ql ~ ) |
| 854 |
are: |
| 855 |
.Bl -tag -width Ds |
| 856 |
.It Cm ~. |
| 857 |
Disconnect. |
| 858 |
.It Cm ~^Z |
| 859 |
Background |
| 860 |
.Nm . |
| 861 |
.It Cm ~# |
| 862 |
List forwarded connections. |
| 863 |
.It Cm ~& |
| 864 |
Background |
| 865 |
.Nm |
| 866 |
at logout when waiting for forwarded connection / X11 sessions to terminate. |
| 867 |
.It Cm ~? |
| 868 |
Display a list of escape characters. |
| 869 |
.It Cm ~B |
| 870 |
Send a BREAK to the remote system |
| 871 |
(only useful for SSH protocol version 2 and if the peer supports it). |
| 872 |
.It Cm ~C |
| 873 |
Open command line. |
| 874 |
Currently this allows the addition of port forwardings using the |
| 875 |
.Fl L |
| 876 |
and |
| 877 |
.Fl R |
| 878 |
options (see above). |
| 879 |
It also allows the cancellation of existing remote port-forwardings |
| 880 |
using |
| 881 |
.Fl KR Ar hostport . |
| 882 |
.Ic !\& Ns Ar command |
| 883 |
allows the user to execute a local command if the |
| 884 |
.Ic PermitLocalCommand |
| 885 |
option is enabled in |
| 886 |
.Xr ssh_config 5 . |
| 887 |
Basic help is available, using the |
| 888 |
.Fl h |
| 889 |
option. |
| 890 |
.It Cm ~R |
| 891 |
Request rekeying of the connection |
| 892 |
(only useful for SSH protocol version 2 and if the peer supports it). |
| 893 |
.El |
| 894 |
.Sh TCP FORWARDING |
| 895 |
Forwarding of arbitrary TCP connections over the secure channel can |
| 896 |
be specified either on the command line or in a configuration file. |
| 897 |
One possible application of TCP forwarding is a secure connection to a |
| 898 |
mail server; another is going through firewalls. |
| 899 |
.Pp |
| 900 |
In the example below, we look at encrypting communication between |
| 901 |
an IRC client and server, even though the IRC server does not directly |
| 902 |
support encrypted communications. |
| 903 |
This works as follows: |
| 904 |
the user connects to the remote host using |
| 905 |
.Nm , |
| 906 |
specifying a port to be used to forward connections |
| 907 |
to the remote server. |
| 908 |
After that it is possible to start the service which is to be encrypted |
| 909 |
on the client machine, |
| 910 |
connecting to the same local port, |
| 911 |
and |
| 912 |
.Nm |
| 913 |
will encrypt and forward the connection. |
| 914 |
.Pp |
| 915 |
The following example tunnels an IRC session from client machine |
| 916 |
.Dq 127.0.0.1 |
| 917 |
(localhost) |
| 918 |
to remote server |
| 919 |
.Dq server.example.com : |
| 920 |
.Bd -literal -offset 4n |
| 921 |
$ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 |
| 922 |
$ irc -c '#users' -p 1234 pinky 127.0.0.1 |
| 923 |
.Ed |
| 924 |
.Pp |
| 925 |
This tunnels a connection to IRC server |
| 926 |
.Dq server.example.com , |
| 927 |
joining channel |
| 928 |
.Dq #users , |
| 929 |
nickname |
| 930 |
.Dq pinky , |
| 931 |
using port 1234. |
| 932 |
It doesn't matter which port is used, |
| 933 |
as long as it's greater than 1023 |
| 934 |
(remember, only root can open sockets on privileged ports) |
| 935 |
and doesn't conflict with any ports already in use. |
| 936 |
The connection is forwarded to port 6667 on the remote server, |
| 937 |
since that's the standard port for IRC services. |
| 938 |
.Pp |
| 939 |
The |
| 940 |
.Fl f |
| 941 |
option backgrounds |
| 942 |
.Nm |
| 943 |
and the remote command |
| 944 |
.Dq sleep 10 |
| 945 |
is specified to allow an amount of time |
| 946 |
(10 seconds, in the example) |
| 947 |
to start the service which is to be tunnelled. |
| 948 |
If no connections are made within the time specified, |
| 949 |
.Nm |
| 950 |
will exit. |
| 951 |
.Sh X11 FORWARDING |
| 952 |
If the |
| 953 |
.Cm ForwardX11 |
| 954 |
variable is set to |
| 955 |
.Dq yes |
| 956 |
(or see the description of the |
| 957 |
.Fl X , |
| 958 |
.Fl x , |
| 959 |
and |
| 960 |
.Fl Y |
| 961 |
options above) |
| 962 |
and the user is using X11 (the |
| 963 |
.Ev DISPLAY |
| 964 |
environment variable is set), the connection to the X11 display is |
| 965 |
automatically forwarded to the remote side in such a way that any X11 |
| 966 |
programs started from the shell (or command) will go through the |
| 967 |
encrypted channel, and the connection to the real X server will be made |
| 968 |
from the local machine. |
| 969 |
The user should not manually set |
| 970 |
.Ev DISPLAY . |
| 971 |
Forwarding of X11 connections can be |
| 972 |
configured on the command line or in configuration files. |
| 973 |
.Pp |
| 974 |
The |
| 975 |
.Ev DISPLAY |
| 976 |
value set by |
| 977 |
.Nm |
| 978 |
will point to the server machine, but with a display number greater than zero. |
| 979 |
This is normal, and happens because |
| 980 |
.Nm |
| 981 |
creates a |
| 982 |
.Dq proxy |
| 983 |
X server on the server machine for forwarding the |
| 984 |
connections over the encrypted channel. |
| 985 |
.Pp |
| 986 |
.Nm |
| 987 |
will also automatically set up Xauthority data on the server machine. |
| 988 |
For this purpose, it will generate a random authorization cookie, |
| 989 |
store it in Xauthority on the server, and verify that any forwarded |
| 990 |
connections carry this cookie and replace it by the real cookie when |
| 991 |
the connection is opened. |
| 992 |
The real authentication cookie is never |
| 993 |
sent to the server machine (and no cookies are sent in the plain). |
| 994 |
.Pp |
| 995 |
If the |
| 996 |
.Cm ForwardAgent |
| 997 |
variable is set to |
| 998 |
.Dq yes |
| 999 |
(or see the description of the |
| 1000 |
.Fl A |
| 1001 |
and |
| 1002 |
.Fl a |
| 1003 |
options above) and |
| 1004 |
the user is using an authentication agent, the connection to the agent |
| 1005 |
is automatically forwarded to the remote side. |
| 1006 |
.Sh VERIFYING HOST KEYS |
| 1007 |
When connecting to a server for the first time, |
| 1008 |
a fingerprint of the server's public key is presented to the user |
| 1009 |
(unless the option |
| 1010 |
.Cm StrictHostKeyChecking |
| 1011 |
has been disabled). |
| 1012 |
Fingerprints can be determined using |
| 1013 |
.Xr ssh-keygen 1 : |
| 1014 |
.Pp |
| 1015 |
.Dl $ ssh-keygen -l -f /usr/etc/ssh_host_rsa_key |
| 1016 |
.Pp |
| 1017 |
If the fingerprint is already known, |
| 1018 |
it can be matched and verified, |
| 1019 |
and the key can be accepted. |
| 1020 |
If the fingerprint is unknown, |
| 1021 |
an alternative method of verification is available: |
| 1022 |
SSH fingerprints verified by DNS. |
| 1023 |
An additional resource record (RR), |
| 1024 |
SSHFP, |
| 1025 |
is added to a zonefile |
| 1026 |
and the connecting client is able to match the fingerprint |
| 1027 |
with that of the key presented. |
| 1028 |
.Pp |
| 1029 |
In this example, we are connecting a client to a server, |
| 1030 |
.Dq host.example.com . |
| 1031 |
The SSHFP resource records should first be added to the zonefile for |
| 1032 |
host.example.com: |
| 1033 |
.Bd -literal -offset indent |
| 1034 |
$ ssh-keygen -f /usr/etc/ssh_host_rsa_key.pub -r host.example.com. |
| 1035 |
$ ssh-keygen -f /usr/etc/ssh_host_dsa_key.pub -r host.example.com. |
| 1036 |
.Ed |
| 1037 |
.Pp |
| 1038 |
The output lines will have to be added to the zonefile. |
| 1039 |
To check that the zone is answering fingerprint queries: |
| 1040 |
.Pp |
| 1041 |
.Dl $ dig -t SSHFP host.example.com |
| 1042 |
.Pp |
| 1043 |
Finally the client connects: |
| 1044 |
.Bd -literal -offset indent |
| 1045 |
$ ssh -o "VerifyHostKeyDNS ask" host.example.com |
| 1046 |
[...] |
| 1047 |
Matching host key fingerprint found in DNS. |
| 1048 |
Are you sure you want to continue connecting (yes/no)? |
| 1049 |
.Ed |
| 1050 |
.Pp |
| 1051 |
See the |
| 1052 |
.Cm VerifyHostKeyDNS |
| 1053 |
option in |
| 1054 |
.Xr ssh_config 5 |
| 1055 |
for more information. |
| 1056 |
.Sh SSH-BASED VIRTUAL PRIVATE NETWORKS |
| 1057 |
.Nm |
| 1058 |
contains support for Virtual Private Network (VPN) tunnelling |
| 1059 |
using the |
| 1060 |
.Xr tun 4 |
| 1061 |
network pseudo-device, |
| 1062 |
allowing two networks to be joined securely. |
| 1063 |
The |
| 1064 |
.Xr sshd_config 5 |
| 1065 |
configuration option |
| 1066 |
.Cm PermitTunnel |
| 1067 |
controls whether the server supports this, |
| 1068 |
and at what level (layer 2 or 3 traffic). |
| 1069 |
.Pp |
| 1070 |
The following example would connect client network 10.0.50.0/24 |
| 1071 |
with remote network 10.0.99.0/24, provided that the SSH server |
| 1072 |
running on the gateway to the remote network, |
| 1073 |
at 192.168.1.15, allows it: |
| 1074 |
.Bd -literal -offset indent |
| 1075 |
# ssh -f -w 0:1 192.168.1.15 true |
| 1076 |
# ifconfig tun0 10.0.50.1 10.0.99.1 netmask 255.255.255.252 |
| 1077 |
.Ed |
| 1078 |
.Pp |
| 1079 |
Client access may be more finely tuned via the |
| 1080 |
.Pa /root/.ssh/authorized_keys |
| 1081 |
file (see below) and the |
| 1082 |
.Cm PermitRootLogin |
| 1083 |
server option. |
| 1084 |
The following entry would permit connections on the first |
| 1085 |
.Xr tun 4 |
| 1086 |
device from user |
| 1087 |
.Dq jane |
| 1088 |
and on the second device from user |
| 1089 |
.Dq john , |
| 1090 |
if |
| 1091 |
.Cm PermitRootLogin |
| 1092 |
is set to |
| 1093 |
.Dq forced-commands-only : |
| 1094 |
.Bd -literal -offset 2n |
| 1095 |
tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane |
| 1096 |
tunnel="2",command="sh /etc/netstart tun1" ssh-rsa ... john |
| 1097 |
.Ed |
| 1098 |
.Pp |
| 1099 |
Since a SSH-based setup entails a fair amount of overhead, |
| 1100 |
it may be more suited to temporary setups, |
| 1101 |
such as for wireless VPNs. |
| 1102 |
More permanent VPNs are better provided by tools such as |
| 1103 |
.Xr ipsecctl 8 |
| 1104 |
and |
| 1105 |
.Xr isakmpd 8 . |
| 1106 |
.Sh ENVIRONMENT |
| 1107 |
.Nm |
| 1108 |
will normally set the following environment variables: |
| 1109 |
.Bl -tag -width "SSH_ORIGINAL_COMMAND" |
| 1110 |
.It Ev DISPLAY |
| 1111 |
The |
| 1112 |
.Ev DISPLAY |
| 1113 |
variable indicates the location of the X11 server. |
| 1114 |
It is automatically set by |
| 1115 |
.Nm |
| 1116 |
to point to a value of the form |
| 1117 |
.Dq hostname:n , |
| 1118 |
where |
| 1119 |
.Dq hostname |
| 1120 |
indicates the host where the shell runs, and |
| 1121 |
.Sq n |
| 1122 |
is an integer \*(Ge 1. |
| 1123 |
.Nm |
| 1124 |
uses this special value to forward X11 connections over the secure |
| 1125 |
channel. |
| 1126 |
The user should normally not set |
| 1127 |
.Ev DISPLAY |
| 1128 |
explicitly, as that |
| 1129 |
will render the X11 connection insecure (and will require the user to |
| 1130 |
manually copy any required authorization cookies). |
| 1131 |
.It Ev HOME |
| 1132 |
Set to the path of the user's home directory. |
| 1133 |
.It Ev LOGNAME |
| 1134 |
Synonym for |
| 1135 |
.Ev USER ; |
| 1136 |
set for compatibility with systems that use this variable. |
| 1137 |
.It Ev MAIL |
| 1138 |
Set to the path of the user's mailbox. |
| 1139 |
.It Ev PATH |
| 1140 |
Set to the default |
| 1141 |
.Ev PATH , |
| 1142 |
as specified when compiling |
| 1143 |
.Nm . |
| 1144 |
.It Ev SSH_ASKPASS |
| 1145 |
If |
| 1146 |
.Nm |
| 1147 |
needs a passphrase, it will read the passphrase from the current |
| 1148 |
terminal if it was run from a terminal. |
| 1149 |
If |
| 1150 |
.Nm |
| 1151 |
does not have a terminal associated with it but |
| 1152 |
.Ev DISPLAY |
| 1153 |
and |
| 1154 |
.Ev SSH_ASKPASS |
| 1155 |
are set, it will execute the program specified by |
| 1156 |
.Ev SSH_ASKPASS |
| 1157 |
and open an X11 window to read the passphrase. |
| 1158 |
This is particularly useful when calling |
| 1159 |
.Nm |
| 1160 |
from a |
| 1161 |
.Pa .xsession |
| 1162 |
or related script. |
| 1163 |
(Note that on some machines it |
| 1164 |
may be necessary to redirect the input from |
| 1165 |
.Pa /dev/null |
| 1166 |
to make this work.) |
| 1167 |
.It Ev SSH_AUTH_SOCK |
| 1168 |
Identifies the path of a |
| 1169 |
.Ux Ns -domain |
| 1170 |
socket used to communicate with the agent. |
| 1171 |
.It Ev SSH_CONNECTION |
| 1172 |
Identifies the client and server ends of the connection. |
| 1173 |
The variable contains |
| 1174 |
four space-separated values: client IP address, client port number, |
| 1175 |
server IP address, and server port number. |
| 1176 |
.It Ev SSH_ORIGINAL_COMMAND |
| 1177 |
This variable contains the original command line if a forced command |
| 1178 |
is executed. |
| 1179 |
It can be used to extract the original arguments. |
| 1180 |
.It Ev SSH_TTY |
| 1181 |
This is set to the name of the tty (path to the device) associated |
| 1182 |
with the current shell or command. |
| 1183 |
If the current session has no tty, |
| 1184 |
this variable is not set. |
| 1185 |
.It Ev TZ |
| 1186 |
This variable is set to indicate the present time zone if it |
| 1187 |
was set when the daemon was started (i.e., the daemon passes the value |
| 1188 |
on to new connections). |
| 1189 |
.It Ev USER |
| 1190 |
Set to the name of the user logging in. |
| 1191 |
.El |
| 1192 |
.Pp |
| 1193 |
Additionally, |
| 1194 |
.Nm |
| 1195 |
reads |
| 1196 |
.Pa ~/.ssh/environment , |
| 1197 |
and adds lines of the format |
| 1198 |
.Dq VARNAME=value |
| 1199 |
to the environment if the file exists and users are allowed to |
| 1200 |
change their environment. |
| 1201 |
For more information, see the |
| 1202 |
.Cm PermitUserEnvironment |
| 1203 |
option in |
| 1204 |
.Xr sshd_config 5 . |
| 1205 |
.Sh FILES |
| 1206 |
.Bl -tag -width Ds -compact |
| 1207 |
.It ~/.rhosts |
| 1208 |
This file is used for host-based authentication (see above). |
| 1209 |
On some machines this file may need to be |
| 1210 |
world-readable if the user's home directory is on an NFS partition, |
| 1211 |
because |
| 1212 |
.Xr sshd 8 |
| 1213 |
reads it as root. |
| 1214 |
Additionally, this file must be owned by the user, |
| 1215 |
and must not have write permissions for anyone else. |
| 1216 |
The recommended |
| 1217 |
permission for most machines is read/write for the user, and not |
| 1218 |
accessible by others. |
| 1219 |
.Pp |
| 1220 |
.It ~/.shosts |
| 1221 |
This file is used in exactly the same way as |
| 1222 |
.Pa .rhosts , |
| 1223 |
but allows host-based authentication without permitting login with |
| 1224 |
rlogin/rsh. |
| 1225 |
.Pp |
| 1226 |
.It ~/.ssh/authorized_keys |
| 1227 |
Lists the public keys (RSA/DSA) that can be used for logging in as this user. |
| 1228 |
The format of this file is described in the |
| 1229 |
.Xr sshd 8 |
| 1230 |
manual page. |
| 1231 |
This file is not highly sensitive, but the recommended |
| 1232 |
permissions are read/write for the user, and not accessible by others. |
| 1233 |
.Pp |
| 1234 |
.It ~/.ssh/config |
| 1235 |
This is the per-user configuration file. |
| 1236 |
The file format and configuration options are described in |
| 1237 |
.Xr ssh_config 5 . |
| 1238 |
Because of the potential for abuse, this file must have strict permissions: |
| 1239 |
read/write for the user, and not accessible by others. |
| 1240 |
It may be group-writable provided that the group in question contains only |
| 1241 |
the user. |
| 1242 |
.Pp |
| 1243 |
.It ~/.ssh/environment |
| 1244 |
Contains additional definitions for environment variables; see |
| 1245 |
.Sx ENVIRONMENT , |
| 1246 |
above. |
| 1247 |
.Pp |
| 1248 |
.It ~/.ssh/identity |
| 1249 |
.It ~/.ssh/id_dsa |
| 1250 |
.It ~/.ssh/id_rsa |
| 1251 |
Contains the private key for authentication. |
| 1252 |
These files |
| 1253 |
contain sensitive data and should be readable by the user but not |
| 1254 |
accessible by others (read/write/execute). |
| 1255 |
.Nm |
| 1256 |
will simply ignore a private key file if it is accessible by others. |
| 1257 |
It is possible to specify a passphrase when |
| 1258 |
generating the key which will be used to encrypt the |
| 1259 |
sensitive part of this file using 3DES. |
| 1260 |
.Pp |
| 1261 |
.It ~/.ssh/identity.pub |
| 1262 |
.It ~/.ssh/id_dsa.pub |
| 1263 |
.It ~/.ssh/id_rsa.pub |
| 1264 |
Contains the public key for authentication. |
| 1265 |
These files are not |
| 1266 |
sensitive and can (but need not) be readable by anyone. |
| 1267 |
.Pp |
| 1268 |
.It ~/.ssh/known_hosts |
| 1269 |
Contains a list of host keys for all hosts the user has logged into |
| 1270 |
that are not already in the systemwide list of known host keys. |
| 1271 |
See |
| 1272 |
.Xr sshd 8 |
| 1273 |
for further details of the format of this file. |
| 1274 |
.Pp |
| 1275 |
.It ~/.ssh/rc |
| 1276 |
Commands in this file are executed by |
| 1277 |
.Nm |
| 1278 |
when the user logs in, just before the user's shell (or command) is |
| 1279 |
started. |
| 1280 |
See the |
| 1281 |
.Xr sshd 8 |
| 1282 |
manual page for more information. |
| 1283 |
.Pp |
| 1284 |
.It /etc/hosts.equiv |
| 1285 |
This file is for host-based authentication (see above). |
| 1286 |
It should only be writable by root. |
| 1287 |
.Pp |
| 1288 |
.It /usr/etc/shosts.equiv |
| 1289 |
This file is used in exactly the same way as |
| 1290 |
.Pa hosts.equiv , |
| 1291 |
but allows host-based authentication without permitting login with |
| 1292 |
rlogin/rsh. |
| 1293 |
.Pp |
| 1294 |
.It Pa /usr/etc/ssh_config |
| 1295 |
Systemwide configuration file. |
| 1296 |
The file format and configuration options are described in |
| 1297 |
.Xr ssh_config 5 . |
| 1298 |
.Pp |
| 1299 |
.It /usr/etc/ssh_host_key |
| 1300 |
.It /usr/etc/ssh_host_dsa_key |
| 1301 |
.It /usr/etc/ssh_host_rsa_key |
| 1302 |
These three files contain the private parts of the host keys |
| 1303 |
and are used for host-based authentication. |
| 1304 |
If protocol version 1 is used, |
| 1305 |
.Nm |
| 1306 |
must be setuid root, since the host key is readable only by root. |
| 1307 |
For protocol version 2, |
| 1308 |
.Nm |
| 1309 |
uses |
| 1310 |
.Xr ssh-keysign 8 |
| 1311 |
to access the host keys, |
| 1312 |
eliminating the requirement that |
| 1313 |
.Nm |
| 1314 |
be setuid root when host-based authentication is used. |
| 1315 |
By default |
| 1316 |
.Nm |
| 1317 |
is not setuid root. |
| 1318 |
.Pp |
| 1319 |
.It /usr/etc/ssh_known_hosts |
| 1320 |
Systemwide list of known host keys. |
| 1321 |
This file should be prepared by the |
| 1322 |
system administrator to contain the public host keys of all machines in the |
| 1323 |
organization. |
| 1324 |
It should be world-readable. |
| 1325 |
See |
| 1326 |
.Xr sshd 8 |
| 1327 |
for further details of the format of this file. |
| 1328 |
.Pp |
| 1329 |
.It /etc/ssh/sshrc |
| 1330 |
Commands in this file are executed by |
| 1331 |
.Nm |
| 1332 |
when the user logs in, just before the user's shell (or command) is started. |
| 1333 |
See the |
| 1334 |
.Xr sshd 8 |
| 1335 |
manual page for more information. |
| 1336 |
.El |
| 1337 |
.Sh SEE ALSO |
| 1338 |
.Xr scp 1 , |
| 1339 |
.Xr sftp 1 , |
| 1340 |
.Xr ssh-add 1 , |
| 1341 |
.Xr ssh-agent 1 , |
| 1342 |
.Xr ssh-argv0 1 , |
| 1343 |
.Xr ssh-keygen 1 , |
| 1344 |
.Xr ssh-keyscan 1 , |
| 1345 |
.Xr tun 4 , |
| 1346 |
.Xr hosts.equiv 5 , |
| 1347 |
.Xr ssh_config 5 , |
| 1348 |
.Xr ssh-keysign 8 , |
| 1349 |
.Xr sshd 8 |
| 1350 |
.Rs |
| 1351 |
.%A T. Ylonen |
| 1352 |
.%A T. Kivinen |
| 1353 |
.%A M. Saarinen |
| 1354 |
.%A T. Rinne |
| 1355 |
.%A S. Lehtinen |
| 1356 |
.%T "SSH Protocol Architecture" |
| 1357 |
.%N draft-ietf-secsh-architecture-12.txt |
| 1358 |
.%D January 2002 |
| 1359 |
.%O work in progress material |
| 1360 |
.Re |
| 1361 |
.Sh AUTHORS |
| 1362 |
OpenSSH is a derivative of the original and free |
| 1363 |
ssh 1.2.12 release by Tatu Ylonen. |
| 1364 |
Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, |
| 1365 |
Theo de Raadt and Dug Song |
| 1366 |
removed many bugs, re-added newer features and |
| 1367 |
created OpenSSH. |
| 1368 |
Markus Friedl contributed the support for SSH |
| 1369 |
protocol versions 1.5 and 2.0. |