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317e5d15 | 1 | SSH(1) OpenBSD Reference Manual SSH(1) |
2 | ||
3 | NAME | |
4 | ssh - OpenSSH SSH client (remote login program) | |
5 | ||
6 | SYNOPSIS | |
7 | ssh [-1246AaCfgkMNnqsTtVvXxY] [-b bind_address] [-c cipher_spec] | |
8 | [-D [bind_address:]port] [-e escape_char] [-F configfile] | |
9 | [-i identity_file] [-L [bind_address:]port:host:hostport] | |
10 | [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port] | |
11 | [-R [bind_address:]port:host:hostport] [-S ctl_path] | |
12 | [-w local_tun[:remote_tun]] [user@]hostname [command] | |
13 | ||
14 | DESCRIPTION | |
15 | ssh (SSH client) is a program for logging into a remote machine and for | |
16 | executing commands on a remote machine. It is intended to replace rlogin | |
17 | and rsh, and provide secure encrypted communications between two untrust- | |
18 | ed hosts over an insecure network. X11 connections and arbitrary TCP | |
19 | ports can also be forwarded over the secure channel. | |
20 | ||
21 | ssh connects and logs into the specified hostname (with optional user | |
22 | name). The user must prove his/her identity to the remote machine using | |
23 | one of several methods depending on the protocol version used (see be- | |
24 | low). | |
25 | ||
26 | If command is specified, it is executed on the remote host instead of a | |
27 | login shell. | |
28 | ||
29 | The options are as follows: | |
30 | ||
31 | -1 Forces ssh to try protocol version 1 only. | |
32 | ||
33 | -2 Forces ssh to try protocol version 2 only. | |
34 | ||
35 | -4 Forces ssh to use IPv4 addresses only. | |
36 | ||
37 | -6 Forces ssh to use IPv6 addresses only. | |
38 | ||
39 | -A Enables forwarding of the authentication agent connection. This | |
40 | can also be specified on a per-host basis in a configuration | |
41 | file. | |
42 | ||
43 | Agent forwarding should be enabled with caution. Users with the | |
44 | ability to bypass file permissions on the remote host (for the | |
45 | agent's Unix-domain socket) can access the local agent through | |
46 | the forwarded connection. An attacker cannot obtain key material | |
47 | from the agent, however they can perform operations on the keys | |
48 | that enable them to authenticate using the identities loaded into | |
49 | the agent. | |
50 | ||
51 | -a Disables forwarding of the authentication agent connection. | |
52 | ||
53 | -b bind_address | |
54 | Use bind_address on the local machine as the source address of | |
55 | the connection. Only useful on systems with more than one ad- | |
56 | dress. | |
57 | ||
58 | -C Requests compression of all data (including stdin, stdout, | |
59 | stderr, and data for forwarded X11 and TCP connections). The | |
60 | compression algorithm is the same used by gzip(1), and the | |
61 | ``level'' can be controlled by the CompressionLevel option for | |
62 | protocol version 1. Compression is desirable on modem lines and | |
63 | other slow connections, but will only slow down things on fast | |
64 | networks. The default value can be set on a host-by-host basis | |
65 | in the configuration files; see the Compression option. | |
66 | ||
67 | -c cipher_spec | |
68 | Selects the cipher specification for encrypting the session. | |
69 | ||
70 | Protocol version 1 allows specification of a single cipher. The | |
71 | supported values are ``3des'', ``blowfish'', and ``des''. 3des | |
72 | (triple-des) is an encrypt-decrypt-encrypt triple with three dif- | |
73 | ferent keys. It is believed to be secure. blowfish is a fast | |
74 | block cipher; it appears very secure and is much faster than | |
75 | 3des. des is only supported in the ssh client for interoperabil- | |
76 | ity with legacy protocol 1 implementations that do not support | |
77 | the 3des cipher. Its use is strongly discouraged due to crypto- | |
78 | graphic weaknesses. The default is ``3des''. | |
79 | ||
80 | For protocol version 2, cipher_spec is a comma-separated list of | |
81 | ciphers listed in order of preference. The supported ciphers | |
82 | are: 3des-cbc, aes128-cbc, aes192-cbc, aes256-cbc, aes128-ctr, | |
83 | aes192-ctr, aes256-ctr, arcfour128, arcfour256, arcfour, blow- | |
84 | fish-cbc, and cast128-cbc. The default is: | |
85 | ||
86 | aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour128, | |
87 | arcfour256,arcfour,aes192-cbc,aes256-cbc,aes128-ctr, | |
88 | aes192-ctr,aes256-ctr | |
89 | ||
90 | -D [bind_address:]port | |
91 | Specifies a local ``dynamic'' application-level port forwarding. | |
92 | This works by allocating a socket to listen to port on the local | |
93 | side, optionally bound to the specified bind_address. Whenever a | |
94 | connection is made to this port, the connection is forwarded over | |
95 | the secure channel, and the application protocol is then used to | |
96 | determine where to connect to from the remote machine. Currently | |
97 | the SOCKS4 and SOCKS5 protocols are supported, and ssh will act | |
98 | as a SOCKS server. Only root can forward privileged ports. Dy- | |
99 | namic port forwardings can also be specified in the configuration | |
100 | file. | |
101 | ||
102 | IPv6 addresses can be specified with an alternative syntax: | |
103 | [bind_address/]port or by enclosing the address in square brack- | |
104 | ets. Only the superuser can forward privileged ports. By de- | |
105 | fault, the local port is bound in accordance with the | |
106 | GatewayPorts setting. However, an explicit bind_address may be | |
107 | used to bind the connection to a specific address. The | |
108 | bind_address of ``localhost'' indicates that the listening port | |
109 | be bound for local use only, while an empty address or `*' indi- | |
110 | cates that the port should be available from all interfaces. | |
111 | ||
112 | -e escape_char | |
113 | Sets the escape character for sessions with a pty (default: `~'). | |
114 | The escape character is only recognized at the beginning of a | |
115 | line. The escape character followed by a dot (`.') closes the | |
116 | connection; followed by control-Z suspends the connection; and | |
117 | followed by itself sends the escape character once. Setting the | |
118 | character to ``none'' disables any escapes and makes the session | |
119 | fully transparent. | |
120 | ||
121 | -F configfile | |
122 | Specifies an alternative per-user configuration file. If a con- | |
123 | figuration file is given on the command line, the system-wide | |
124 | configuration file (/etc/ssh/ssh_config) will be ignored. The | |
125 | default for the per-user configuration file is ~/.ssh/config. | |
126 | ||
127 | -f Requests ssh to go to background just before command execution. | |
128 | This is useful if ssh is going to ask for passwords or passphras- | |
129 | es, but the user wants it in the background. This implies -n. | |
130 | The recommended way to start X11 programs at a remote site is | |
131 | with something like ssh -f host xterm. | |
132 | ||
133 | -g Allows remote hosts to connect to local forwarded ports. | |
134 | ||
135 | -I smartcard_device | |
136 | Specify the device ssh should use to communicate with a smartcard | |
137 | used for storing the user's private RSA key. This option is only | |
138 | available if support for smartcard devices is compiled in (de- | |
139 | fault is no support). | |
140 | ||
141 | -i identity_file | |
142 | Selects a file from which the identity (private key) for RSA or | |
143 | DSA authentication is read. The default is ~/.ssh/identity for | |
144 | protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for pro- | |
145 | tocol version 2. Identity files may also be specified on a per- | |
146 | host basis in the configuration file. It is possible to have | |
147 | multiple -i options (and multiple identities specified in config- | |
148 | uration files). | |
149 | ||
150 | -k Disables forwarding (delegation) of GSSAPI credentials to the | |
151 | server. | |
152 | ||
153 | -L [bind_address:]port:host:hostport | |
154 | Specifies that the given port on the local (client) host is to be | |
155 | forwarded to the given host and port on the remote side. This | |
156 | works by allocating a socket to listen to port on the local side, | |
157 | optionally bound to the specified bind_address. Whenever a con- | |
158 | nection is made to this port, the connection is forwarded over | |
159 | the secure channel, and a connection is made to host port | |
160 | hostport from the remote machine. Port forwardings can also be | |
161 | specified in the configuration file. IPv6 addresses can be spec- | |
162 | ified with an alternative syntax: [bind_address/]port/host/host- | |
163 | port or by enclosing the address in square brackets. Only the | |
164 | superuser can forward privileged ports. By default, the local | |
165 | port is bound in accordance with the GatewayPorts setting. How- | |
166 | ever, an explicit bind_address may be used to bind the connection | |
167 | to a specific address. The bind_address of ``localhost'' indi- | |
168 | cates that the listening port be bound for local use only, while | |
169 | an empty address or `*' indicates that the port should be avail- | |
170 | able from all interfaces. | |
171 | ||
172 | -l login_name | |
173 | Specifies the user to log in as on the remote machine. This also | |
174 | may be specified on a per-host basis in the configuration file. | |
175 | ||
176 | -M Places the ssh client into ``master'' mode for connection shar- | |
177 | ing. Multiple -M options places ssh into ``master'' mode with | |
178 | confirmation required before slave connections are accepted. Re- | |
179 | fer to the description of ControlMaster in ssh_config(5) for de- | |
180 | tails. | |
181 | ||
182 | -m mac_spec | |
183 | Additionally, for protocol version 2 a comma-separated list of | |
184 | MAC (message authentication code) algorithms can be specified in | |
185 | order of preference. See the MACs keyword for more information. | |
186 | ||
187 | -N Do not execute a remote command. This is useful for just for- | |
188 | warding ports (protocol version 2 only). | |
189 | ||
190 | -n Redirects stdin from /dev/null (actually, prevents reading from | |
191 | stdin). This must be used when ssh is run in the background. A | |
192 | common trick is to use this to run X11 programs on a remote ma- | |
193 | chine. For example, ssh -n shadows.cs.hut.fi emacs & will start | |
194 | an emacs on shadows.cs.hut.fi, and the X11 connection will be au- | |
195 | tomatically forwarded over an encrypted channel. The ssh program | |
196 | will be put in the background. (This does not work if ssh needs | |
197 | to ask for a password or passphrase; see also the -f option.) | |
198 | ||
199 | -O ctl_cmd | |
200 | Control an active connection multiplexing master process. When | |
201 | the -O option is specified, the ctl_cmd argument is interpreted | |
202 | and passed to the master process. Valid commands are: ``check'' | |
203 | (check that the master process is running) and ``exit'' (request | |
204 | the master to exit). | |
205 | ||
206 | -o option | |
207 | Can be used to give options in the format used in the configura- | |
208 | tion file. This is useful for specifying options for which there | |
209 | is no separate command-line flag. For full details of the op- | |
210 | tions listed below, and their possible values, see ssh_config(5). | |
211 | ||
212 | AddressFamily | |
213 | BatchMode | |
214 | BindAddress | |
215 | ChallengeResponseAuthentication | |
216 | CheckHostIP | |
217 | Cipher | |
218 | Ciphers | |
219 | ClearAllForwardings | |
220 | Compression | |
221 | CompressionLevel | |
222 | ConnectionAttempts | |
223 | ConnectTimeout | |
224 | ControlMaster | |
225 | ControlPath | |
226 | DynamicForward | |
227 | EscapeChar | |
228 | ExitOnForwardFailure | |
229 | ForwardAgent | |
230 | ForwardX11 | |
231 | ForwardX11Trusted | |
232 | GatewayPorts | |
233 | GlobalKnownHostsFile | |
234 | GSSAPIAuthentication | |
235 | GSSAPIDelegateCredentials | |
236 | HashKnownHosts | |
237 | Host | |
238 | HostbasedAuthentication | |
239 | HostKeyAlgorithms | |
240 | HostKeyAlias | |
241 | HostName | |
242 | IdentityFile | |
243 | IdentitiesOnly | |
244 | KbdInteractiveDevices | |
245 | LocalCommand | |
246 | LocalForward | |
247 | LogLevel | |
248 | MACs | |
249 | NoHostAuthenticationForLocalhost | |
250 | NumberOfPasswordPrompts | |
251 | PasswordAuthentication | |
252 | PermitLocalCommand | |
253 | Port | |
254 | PreferredAuthentications | |
255 | Protocol | |
256 | ProxyCommand | |
257 | PubkeyAuthentication | |
258 | RekeyLimit | |
259 | RemoteForward | |
260 | RhostsRSAAuthentication | |
261 | RSAAuthentication | |
262 | SendEnv | |
263 | ServerAliveInterval | |
264 | ServerAliveCountMax | |
265 | SmartcardDevice | |
266 | StrictHostKeyChecking | |
267 | TCPKeepAlive | |
268 | Tunnel | |
269 | TunnelDevice | |
270 | UsePrivilegedPort | |
271 | User | |
272 | UserKnownHostsFile | |
273 | VerifyHostKeyDNS | |
274 | XAuthLocation | |
275 | ||
276 | -p port | |
277 | Port to connect to on the remote host. This can be specified on | |
278 | a per-host basis in the configuration file. | |
279 | ||
280 | -q Quiet mode. Causes all warning and diagnostic messages to be | |
281 | suppressed. | |
282 | ||
283 | -R [bind_address:]port:host:hostport | |
284 | Specifies that the given port on the remote (server) host is to | |
285 | be forwarded to the given host and port on the local side. This | |
286 | works by allocating a socket to listen to port on the remote | |
287 | side, and whenever a connection is made to this port, the connec- | |
288 | tion is forwarded over the secure channel, and a connection is | |
289 | made to host port hostport from the local machine. | |
290 | ||
291 | Port forwardings can also be specified in the configuration file. | |
292 | Privileged ports can be forwarded only when logging in as root on | |
293 | the remote machine. IPv6 addresses can be specified by enclosing | |
294 | the address in square braces or using an alternative syntax: | |
295 | [bind_address/]host/port/hostport. | |
296 | ||
297 | By default, the listening socket on the server will be bound to | |
298 | the loopback interface only. This may be overriden by specifying | |
299 | a bind_address. An empty bind_address, or the address `*', indi- | |
300 | cates that the remote socket should listen on all interfaces. | |
301 | Specifying a remote bind_address will only succeed if the serv- | |
302 | er's GatewayPorts option is enabled (see sshd_config(5)). | |
303 | ||
304 | -S ctl_path | |
305 | Specifies the location of a control socket for connection shar- | |
306 | ing. Refer to the description of ControlPath and ControlMaster | |
307 | in ssh_config(5) for details. | |
308 | ||
309 | -s May be used to request invocation of a subsystem on the remote | |
310 | system. Subsystems are a feature of the SSH2 protocol which fa- | |
311 | cilitate the use of SSH as a secure transport for other applica- | |
312 | tions (eg. sftp(1)). The subsystem is specified as the remote | |
313 | command. | |
314 | ||
315 | -T Disable pseudo-tty allocation. | |
316 | ||
317 | -t Force pseudo-tty allocation. This can be used to execute arbi- | |
318 | trary screen-based programs on a remote machine, which can be | |
319 | very useful, e.g. when implementing menu services. Multiple -t | |
320 | options force tty allocation, even if ssh has no local tty. | |
321 | ||
322 | -V Display the version number and exit. | |
323 | ||
324 | -v Verbose mode. Causes ssh to print debugging messages about its | |
325 | progress. This is helpful in debugging connection, authentica- | |
326 | tion, and configuration problems. Multiple -v options increase | |
327 | the verbosity. The maximum is 3. | |
328 | ||
329 | -w local_tun[:remote_tun] | |
330 | Requests tunnel device forwarding with the specified tun(4) de- | |
331 | vices between the client (local_tun) and the server (remote_tun). | |
332 | ||
333 | The devices may be specified by numerical ID or the keyword | |
334 | ``any'', which uses the next available tunnel device. If | |
335 | remote_tun is not specified, it defaults to ``any''. See also | |
336 | the Tunnel and TunnelDevice directives in ssh_config(5). If the | |
337 | Tunnel directive is unset, it is set to the default tunnel mode, | |
338 | which is ``point-to-point''. | |
339 | ||
340 | -X Enables X11 forwarding. This can also be specified on a per-host | |
341 | basis in a configuration file. | |
342 | ||
343 | X11 forwarding should be enabled with caution. Users with the | |
344 | ability to bypass file permissions on the remote host (for the | |
345 | user's X authorization database) can access the local X11 display | |
346 | through the forwarded connection. An attacker may then be able | |
347 | to perform activities such as keystroke monitoring. | |
348 | ||
349 | For this reason, X11 forwarding is subjected to X11 SECURITY ex- | |
350 | tension restrictions by default. Please refer to the ssh -Y op- | |
351 | tion and the ForwardX11Trusted directive in ssh_config(5) for | |
352 | more information. | |
353 | ||
354 | -x Disables X11 forwarding. | |
355 | ||
356 | -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not | |
357 | subjected to the X11 SECURITY extension controls. | |
358 | ||
359 | ssh may additionally obtain configuration data from a per-user configura- | |
360 | tion file and a system-wide configuration file. The file format and con- | |
361 | figuration options are described in ssh_config(5). | |
362 | ||
363 | ssh exits with the exit status of the remote command or with 255 if an | |
364 | error occurred. | |
365 | ||
366 | AUTHENTICATION | |
367 | The OpenSSH SSH client supports SSH protocols 1 and 2. Protocol 2 is the | |
368 | default, with ssh falling back to protocol 1 if it detects protocol 2 is | |
369 | unsupported. These settings may be altered using the Protocol option in | |
370 | ssh_config(5), or enforced using the -1 and -2 options (see above). Both | |
371 | protocols support similar authentication methods, but protocol 2 is pre- | |
372 | ferred since it provides additional mechanisms for confidentiality (the | |
373 | traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and | |
374 | integrity (hmac-md5, hmac-sha1, hmac-ripemd160). Protocol 1 lacks a | |
375 | strong mechanism for ensuring the integrity of the connection. | |
376 | ||
377 | The methods available for authentication are: GSSAPI-based authentica- | |
378 | tion, host-based authentication, public key authentication, challenge-re- | |
379 | sponse authentication, and password authentication. Authentication meth- | |
380 | ods are tried in the order specified above, though protocol 2 has a con- | |
381 | figuration option to change the default order: PreferredAuthentications. | |
382 | ||
383 | Host-based authentication works as follows: If the machine the user logs | |
384 | in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote | |
385 | machine, and the user names are the same on both sides, or if the files | |
386 | ~/.rhosts or ~/.shosts exist in the user's home directory on the remote | |
387 | machine and contain a line containing the name of the client machine and | |
388 | the name of the user on that machine, the user is considered for login. | |
389 | Additionally, the server must be able to verify the client's host key | |
390 | (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, | |
391 | below) for login to be permitted. This authentication method closes se- | |
392 | curity holes due to IP spoofing, DNS spoofing, and routing spoofing. | |
393 | [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the | |
394 | rlogin/rsh protocol in general, are inherently insecure and should be | |
395 | disabled if security is desired.] | |
396 | ||
397 | Public key authentication works as follows: The scheme is based on pub- | |
398 | lic-key cryptography, using cryptosystems where encryption and decryption | |
399 | are done using separate keys, and it is unfeasible to derive the decryp- | |
400 | tion key from the encryption key. The idea is that each user creates a | |
401 | public/private key pair for authentication purposes. The server knows | |
402 | the public key, and only the user knows the private key. ssh implements | |
403 | public key authentication protocol automatically, using either the RSA or | |
404 | DSA algorithms. Protocol 1 is restricted to using only RSA keys, but | |
405 | protocol 2 may use either. The HISTORY section of ssl(8) contains a | |
406 | brief discussion of the two algorithms. | |
407 | ||
408 | The file ~/.ssh/authorized_keys lists the public keys that are permitted | |
409 | for logging in. When the user logs in, the ssh program tells the server | |
410 | which key pair it would like to use for authentication. The client | |
411 | proves that it has access to the private key and the server checks that | |
412 | the corresponding public key is authorized to accept the account. | |
413 | ||
414 | The user creates his/her key pair by running ssh-keygen(1). This stores | |
415 | the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol | |
416 | 2 DSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in | |
417 | ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), or | |
418 | ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home directory. The us- | |
419 | er should then copy the public key to ~/.ssh/authorized_keys in his/her | |
420 | home directory on the remote machine. The authorized_keys file corre- | |
421 | sponds to the conventional ~/.rhosts file, and has one key per line, | |
422 | though the lines can be very long. After this, the user can log in with- | |
423 | out giving the password. | |
424 | ||
425 | The most convenient way to use public key authentication may be with an | |
426 | authentication agent. See ssh-agent(1) for more information. | |
427 | ||
428 | Challenge-response authentication works as follows: The server sends an | |
429 | arbitrary "challenge" text, and prompts for a response. Protocol 2 al- | |
430 | lows multiple challenges and responses; protocol 1 is restricted to just | |
431 | one challenge/response. Examples of challenge-response authentication | |
432 | include BSD Authentication (see login.conf(5)) and PAM (some non-OpenBSD | |
433 | systems). | |
434 | ||
435 | Finally, if other authentication methods fail, ssh prompts the user for a | |
436 | password. The password is sent to the remote host for checking; however, | |
437 | since all communications are encrypted, the password cannot be seen by | |
438 | someone listening on the network. | |
439 | ||
440 | ssh automatically maintains and checks a database containing identifica- | |
441 | tion for all hosts it has ever been used with. Host keys are stored in | |
442 | ~/.ssh/known_hosts in the user's home directory. Additionally, the file | |
443 | /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any | |
444 | new hosts are automatically added to the user's file. If a host's iden- | |
445 | tification ever changes, ssh warns about this and disables password au- | |
446 | thentication to prevent server spoofing or man-in-the-middle attacks, | |
447 | which could otherwise be used to circumvent the encryption. The | |
448 | StrictHostKeyChecking option can be used to control logins to machines | |
449 | whose host key is not known or has changed. | |
450 | ||
451 | When the user's identity has been accepted by the server, the server ei- | |
452 | ther executes the given command, or logs into the machine and gives the | |
453 | user a normal shell on the remote machine. All communication with the | |
454 | remote command or shell will be automatically encrypted. | |
455 | ||
456 | If a pseudo-terminal has been allocated (normal login session), the user | |
457 | may use the escape characters noted below. | |
458 | ||
459 | If no pseudo-tty has been allocated, the session is transparent and can | |
460 | be used to reliably transfer binary data. On most systems, setting the | |
461 | escape character to ``none'' will also make the session transparent even | |
462 | if a tty is used. | |
463 | ||
464 | The session terminates when the command or shell on the remote machine | |
465 | exits and all X11 and TCP connections have been closed. | |
466 | ||
467 | ESCAPE CHARACTERS | |
468 | When a pseudo-terminal has been requested, ssh supports a number of func- | |
469 | tions through the use of an escape character. | |
470 | ||
471 | A single tilde character can be sent as ~~ or by following the tilde by a | |
472 | character other than those described below. The escape character must | |
473 | always follow a newline to be interpreted as special. The escape charac- | |
474 | ter can be changed in configuration files using the EscapeChar configura- | |
475 | tion directive or on the command line by the -e option. | |
476 | ||
477 | The supported escapes (assuming the default `~') are: | |
478 | ||
479 | ~. Disconnect. | |
480 | ||
481 | ~^Z Background ssh. | |
482 | ||
483 | ~# List forwarded connections. | |
484 | ||
485 | ~& Background ssh at logout when waiting for forwarded connection / | |
486 | X11 sessions to terminate. | |
487 | ||
488 | ~? Display a list of escape characters. | |
489 | ||
490 | ~B Send a BREAK to the remote system (only useful for SSH protocol | |
491 | version 2 and if the peer supports it). | |
492 | ||
493 | ~C Open command line. Currently this allows the addition of port | |
494 | forwardings using the -L and -R options (see above). It also al- | |
495 | lows the cancellation of existing remote port-forwardings using | |
496 | -KR[bind_address:]port. !command allows the user to execute a | |
497 | local command if the PermitLocalCommand option is enabled in | |
498 | ssh_config(5). Basic help is available, using the -h option. | |
499 | ||
500 | ~R Request rekeying of the connection (only useful for SSH protocol | |
501 | version 2 and if the peer supports it). | |
502 | ||
503 | TCP FORWARDING | |
504 | Forwarding of arbitrary TCP connections over the secure channel can be | |
505 | specified either on the command line or in a configuration file. One | |
506 | possible application of TCP forwarding is a secure connection to a mail | |
507 | server; another is going through firewalls. | |
508 | ||
509 | In the example below, we look at encrypting communication between an IRC | |
510 | client and server, even though the IRC server does not directly support | |
511 | encrypted communications. This works as follows: the user connects to | |
512 | the remote host using ssh, specifying a port to be used to forward con- | |
513 | nections to the remote server. After that it is possible to start the | |
514 | service which is to be encrypted on the client machine, connecting to the | |
515 | same local port, and ssh will encrypt and forward the connection. | |
516 | ||
517 | The following example tunnels an IRC session from client machine | |
518 | ``127.0.0.1'' (localhost) to remote server ``server.example.com'': | |
519 | ||
520 | $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 | |
521 | $ irc -c '#users' -p 1234 pinky 127.0.0.1 | |
522 | ||
523 | This tunnels a connection to IRC server ``server.example.com'', joining | |
524 | channel ``#users'', nickname ``pinky'', using port 1234. It doesn't mat- | |
525 | ter which port is used, as long as it's greater than 1023 (remember, only | |
526 | root can open sockets on privileged ports) and doesn't conflict with any | |
527 | ports already in use. The connection is forwarded to port 6667 on the | |
528 | remote server, since that's the standard port for IRC services. | |
529 | ||
530 | The -f option backgrounds ssh and the remote command ``sleep 10'' is | |
531 | specified to allow an amount of time (10 seconds, in the example) to | |
532 | start the service which is to be tunnelled. If no connections are made | |
533 | within the time specified, ssh will exit. | |
534 | ||
535 | X11 FORWARDING | |
536 | If the ForwardX11 variable is set to ``yes'' (or see the description of | |
537 | the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY | |
538 | environment variable is set), the connection to the X11 display is auto- | |
539 | matically forwarded to the remote side in such a way that any X11 pro- | |
540 | grams started from the shell (or command) will go through the encrypted | |
541 | channel, and the connection to the real X server will be made from the | |
542 | local machine. The user should not manually set DISPLAY. Forwarding of | |
543 | X11 connections can be configured on the command line or in configuration | |
544 | files. | |
545 | ||
546 | The DISPLAY value set by ssh will point to the server machine, but with a | |
547 | display number greater than zero. This is normal, and happens because | |
548 | ssh creates a ``proxy'' X server on the server machine for forwarding the | |
549 | connections over the encrypted channel. | |
550 | ||
551 | ssh will also automatically set up Xauthority data on the server machine. | |
552 | For this purpose, it will generate a random authorization cookie, store | |
553 | it in Xauthority on the server, and verify that any forwarded connections | |
554 | carry this cookie and replace it by the real cookie when the connection | |
555 | is opened. The real authentication cookie is never sent to the server | |
556 | machine (and no cookies are sent in the plain). | |
557 | ||
558 | If the ForwardAgent variable is set to ``yes'' (or see the description of | |
559 | the -A and -a options above) and the user is using an authentication | |
560 | agent, the connection to the agent is automatically forwarded to the re- | |
561 | mote side. | |
562 | ||
563 | VERIFYING HOST KEYS | |
564 | When connecting to a server for the first time, a fingerprint of the | |
565 | server's public key is presented to the user (unless the option | |
566 | StrictHostKeyChecking has been disabled). Fingerprints can be determined | |
567 | using ssh-keygen(1): | |
568 | ||
569 | $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key | |
570 | ||
571 | If the fingerprint is already known, it can be matched and verified, and | |
572 | the key can be accepted. If the fingerprint is unknown, an alternative | |
573 | method of verification is available: SSH fingerprints verified by DNS. | |
574 | An additional resource record (RR), SSHFP, is added to a zonefile and the | |
575 | connecting client is able to match the fingerprint with that of the key | |
576 | presented. | |
577 | ||
578 | In this example, we are connecting a client to a server, | |
579 | ``host.example.com''. The SSHFP resource records should first be added | |
580 | to the zonefile for host.example.com: | |
581 | ||
582 | $ ssh-keygen -r host.example.com. | |
583 | ||
584 | The output lines will have to be added to the zonefile. To check that | |
585 | the zone is answering fingerprint queries: | |
586 | ||
587 | $ dig -t SSHFP host.example.com | |
588 | ||
589 | Finally the client connects: | |
590 | ||
591 | $ ssh -o "VerifyHostKeyDNS ask" host.example.com | |
592 | [...] | |
593 | Matching host key fingerprint found in DNS. | |
594 | Are you sure you want to continue connecting (yes/no)? | |
595 | ||
596 | See the VerifyHostKeyDNS option in ssh_config(5) for more information. | |
597 | ||
598 | SSH-BASED VIRTUAL PRIVATE NETWORKS | |
599 | ssh contains support for Virtual Private Network (VPN) tunnelling using | |
600 | the tun(4) network pseudo-device, allowing two networks to be joined se- | |
601 | curely. The sshd_config(5) configuration option PermitTunnel controls | |
602 | whether the server supports this, and at what level (layer 2 or 3 traf- | |
603 | fic). | |
604 | ||
605 | The following example would connect client network 10.0.50.0/24 with re- | |
606 | mote network 10.0.99.0/24, provided that the SSH server running on the | |
607 | gateway to the remote network, at 192.168.1.15, allows it: | |
608 | ||
609 | # ssh -f -w 0:1 192.168.1.15 true | |
610 | # ifconfig tun0 10.0.50.1 10.0.99.1 netmask 255.255.255.252 | |
611 | ||
612 | Client access may be more finely tuned via the /root/.ssh/authorized_keys | |
613 | file (see below) and the PermitRootLogin server option. The following | |
614 | entry would permit connections on tun(4) device 1 from user ``jane'' and | |
615 | on tun device 2 from user ``john'', if PermitRootLogin is set to | |
616 | ``forced-commands-only'': | |
617 | ||
618 | tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane | |
619 | tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john | |
620 | ||
621 | Since a SSH-based setup entails a fair amount of overhead, it may be more | |
622 | suited to temporary setups, such as for wireless VPNs. More permanent | |
623 | VPNs are better provided by tools such as ipsecctl(8) and isakmpd(8). | |
624 | ||
625 | ENVIRONMENT | |
626 | ssh will normally set the following environment variables: | |
627 | ||
628 | DISPLAY The DISPLAY variable indicates the location of the | |
629 | X11 server. It is automatically set by ssh to | |
630 | point to a value of the form ``hostname:n'', where | |
631 | ``hostname'' indicates the host where the shell | |
632 | runs, and `n' is an integer >= 1. ssh uses this | |
633 | special value to forward X11 connections over the | |
634 | secure channel. The user should normally not set | |
635 | DISPLAY explicitly, as that will render the X11 | |
636 | connection insecure (and will require the user to | |
637 | manually copy any required authorization cookies). | |
638 | ||
639 | HOME Set to the path of the user's home directory. | |
640 | ||
641 | LOGNAME Synonym for USER; set for compatibility with sys- | |
642 | tems that use this variable. | |
643 | ||
644 | MAIL Set to the path of the user's mailbox. | |
645 | ||
646 | PATH Set to the default PATH, as specified when compil- | |
647 | ing ssh. | |
648 | ||
649 | SSH_ASKPASS If ssh needs a passphrase, it will read the | |
650 | passphrase from the current terminal if it was run | |
651 | from a terminal. If ssh does not have a terminal | |
652 | associated with it but DISPLAY and SSH_ASKPASS are | |
653 | set, it will execute the program specified by | |
654 | SSH_ASKPASS and open an X11 window to read the | |
655 | passphrase. This is particularly useful when call- | |
656 | ing ssh from a .xsession or related script. (Note | |
657 | that on some machines it may be necessary to redi- | |
658 | rect the input from /dev/null to make this work.) | |
659 | ||
660 | SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to | |
661 | communicate with the agent. | |
662 | ||
663 | SSH_CONNECTION Identifies the client and server ends of the con- | |
664 | nection. The variable contains four space-separat- | |
665 | ed values: client IP address, client port number, | |
666 | server IP address, and server port number. | |
667 | ||
668 | SSH_ORIGINAL_COMMAND This variable contains the original command line if | |
669 | a forced command is executed. It can be used to | |
670 | extract the original arguments. | |
671 | ||
672 | SSH_TTY This is set to the name of the tty (path to the de- | |
673 | vice) associated with the current shell or command. | |
674 | If the current session has no tty, this variable is | |
675 | not set. | |
676 | ||
677 | TZ This variable is set to indicate the present time | |
678 | zone if it was set when the daemon was started | |
679 | (i.e. the daemon passes the value on to new connec- | |
680 | tions). | |
681 | ||
682 | USER Set to the name of the user logging in. | |
683 | ||
684 | Additionally, ssh reads ~/.ssh/environment, and adds lines of the format | |
685 | ``VARNAME=value'' to the environment if the file exists and users are al- | |
686 | lowed to change their environment. For more information, see the | |
687 | PermitUserEnvironment option in sshd_config(5). | |
688 | ||
689 | FILES | |
690 | ~/.rhosts | |
691 | This file is used for host-based authentication (see above). On | |
692 | some machines this file may need to be world-readable if the us- | |
693 | er's home directory is on an NFS partition, because sshd(8) reads | |
694 | it as root. Additionally, this file must be owned by the user, | |
695 | and must not have write permissions for anyone else. The recom- | |
696 | mended permission for most machines is read/write for the user, | |
697 | and not accessible by others. | |
698 | ||
699 | ~/.shosts | |
700 | This file is used in exactly the same way as .rhosts, but allows | |
701 | host-based authentication without permitting login with | |
702 | rlogin/rsh. | |
703 | ||
704 | ~/.ssh/authorized_keys | |
705 | Lists the public keys (RSA/DSA) that can be used for logging in | |
706 | as this user. The format of this file is described in the | |
707 | sshd(8) manual page. This file is not highly sensitive, but the | |
708 | recommended permissions are read/write for the user, and not ac- | |
709 | cessible by others. | |
710 | ||
711 | ~/.ssh/config | |
712 | This is the per-user configuration file. The file format and | |
713 | configuration options are described in ssh_config(5). Because of | |
714 | the potential for abuse, this file must have strict permissions: | |
715 | read/write for the user, and not accessible by others. | |
716 | ||
717 | ~/.ssh/environment | |
718 | Contains additional definitions for environment variables; see | |
719 | ENVIRONMENT, above. | |
720 | ||
721 | ~/.ssh/identity | |
722 | ~/.ssh/id_dsa | |
723 | ~/.ssh/id_rsa | |
724 | Contains the private key for authentication. These files contain | |
725 | sensitive data and should be readable by the user but not acces- | |
726 | sible by others (read/write/execute). ssh will simply ignore a | |
727 | private key file if it is accessible by others. It is possible | |
728 | to specify a passphrase when generating the key which will be | |
729 | used to encrypt the sensitive part of this file using 3DES. | |
730 | ||
731 | ~/.ssh/identity.pub | |
732 | ~/.ssh/id_dsa.pub | |
733 | ~/.ssh/id_rsa.pub | |
734 | Contains the public key for authentication. These files are not | |
735 | sensitive and can (but need not) be readable by anyone. | |
736 | ||
737 | ~/.ssh/known_hosts | |
738 | Contains a list of host keys for all hosts the user has logged | |
739 | into that are not already in the systemwide list of known host | |
740 | keys. See sshd(8) for further details of the format of this | |
741 | file. | |
742 | ||
743 | ~/.ssh/rc | |
744 | Commands in this file are executed by ssh when the user logs in, | |
745 | just before the user's shell (or command) is started. See the | |
746 | sshd(8) manual page for more information. | |
747 | ||
748 | /etc/hosts.equiv | |
749 | This file is for host-based authentication (see above). It | |
750 | should only be writable by root. | |
751 | ||
752 | /etc/shosts.equiv | |
753 | This file is used in exactly the same way as hosts.equiv, but al- | |
754 | lows host-based authentication without permitting login with | |
755 | rlogin/rsh. | |
756 | ||
757 | /etc/ssh/ssh_config | |
758 | Systemwide configuration file. The file format and configuration | |
759 | options are described in ssh_config(5). | |
760 | ||
761 | /etc/ssh/ssh_host_key | |
762 | /etc/ssh/ssh_host_dsa_key | |
763 | /etc/ssh/ssh_host_rsa_key | |
764 | These three files contain the private parts of the host keys and | |
765 | are used for host-based authentication. If protocol version 1 is | |
766 | used, ssh must be setuid root, since the host key is readable on- | |
767 | ly by root. For protocol version 2, ssh uses ssh-keysign(8) to | |
768 | access the host keys, eliminating the requirement that ssh be se- | |
769 | tuid root when host-based authentication is used. By default ssh | |
770 | is not setuid root. | |
771 | ||
772 | /etc/ssh/ssh_known_hosts | |
773 | Systemwide list of known host keys. This file should be prepared | |
774 | by the system administrator to contain the public host keys of | |
775 | all machines in the organization. It should be world-readable. | |
776 | See sshd(8) for further details of the format of this file. | |
777 | ||
778 | /etc/ssh/sshrc | |
779 | Commands in this file are executed by ssh when the user logs in, | |
780 | just before the user's shell (or command) is started. See the | |
781 | sshd(8) manual page for more information. | |
782 | ||
783 | SEE ALSO | |
784 | scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), | |
785 | tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8) | |
786 | ||
787 | The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006. | |
788 | ||
789 | The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006. | |
790 | ||
791 | The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006. | |
792 | ||
793 | The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006. | |
794 | ||
795 | The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006. | |
796 | ||
797 | Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC | |
798 | 4255, 2006. | |
799 | ||
800 | Generic Message Exchange Authentication for the Secure Shell Protocol | |
801 | (SSH), RFC 4256, 2006. | |
802 | ||
803 | The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006. | |
804 | ||
805 | The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006. | |
806 | ||
807 | Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer | |
808 | Protocol, RFC 4345, 2006. | |
809 | ||
810 | Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer | |
811 | Protocol, RFC 4419, 2006. | |
812 | ||
813 | AUTHORS | |
814 | OpenSSH is a derivative of the original and free ssh 1.2.12 release by | |
815 | Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo | |
816 | de Raadt and Dug Song removed many bugs, re-added newer features and | |
817 | created OpenSSH. Markus Friedl contributed the support for SSH protocol | |
818 | versions 1.5 and 2.0. | |
819 | ||
820 | OpenBSD 4.0 September 25, 1999 13 |