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1 | SSH(1) OpenBSD Reference Manual SSH(1) | ||
2 | |||
3 | NAME | ||
4 | ssh - OpenSSH SSH client (remote login program) | ||
5 | |||
6 | SYNOPSIS | ||
7 | ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec] | ||
8 | [-D [bind_address:]port] [-e escape_char] [-F configfile] [-I pkcs11] | ||
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] [-W host:port] | ||
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 | ||
18 | untrusted hosts over an insecure network. X11 connections and arbitrary | ||
19 | TCP 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 | ||
24 | below). | ||
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 | ||
56 | address. | ||
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 | ||
73 | different 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 | ||
76 | interoperability with legacy protocol 1 implementations that do | ||
77 | not support the 3des cipher. Its use is strongly discouraged due | ||
78 | to cryptographic 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. See the Ciphers keyword | ||
82 | in ssh_config(5) for more information. | ||
83 | |||
84 | -D [bind_address:]port | ||
85 | Specifies a local ``dynamic'' application-level port forwarding. | ||
86 | This works by allocating a socket to listen to port on the local | ||
87 | side, optionally bound to the specified bind_address. Whenever a | ||
88 | connection is made to this port, the connection is forwarded over | ||
89 | the secure channel, and the application protocol is then used to | ||
90 | determine where to connect to from the remote machine. Currently | ||
91 | the SOCKS4 and SOCKS5 protocols are supported, and ssh will act | ||
92 | as a SOCKS server. Only root can forward privileged ports. | ||
93 | Dynamic port forwardings can also be specified in the | ||
94 | configuration file. | ||
95 | |||
96 | IPv6 addresses can be specified by enclosing the address in | ||
97 | square brackets. Only the superuser can forward privileged | ||
98 | ports. By default, the local port is bound in accordance with | ||
99 | the GatewayPorts setting. However, an explicit bind_address may | ||
100 | be used to bind the connection to a specific address. The | ||
101 | bind_address of ``localhost'' indicates that the listening port | ||
102 | be bound for local use only, while an empty address or `*' | ||
103 | indicates that the port should be available from all interfaces. | ||
104 | |||
105 | -e escape_char | ||
106 | Sets the escape character for sessions with a pty (default: `~'). | ||
107 | The escape character is only recognized at the beginning of a | ||
108 | line. The escape character followed by a dot (`.') closes the | ||
109 | connection; followed by control-Z suspends the connection; and | ||
110 | followed by itself sends the escape character once. Setting the | ||
111 | character to ``none'' disables any escapes and makes the session | ||
112 | fully transparent. | ||
113 | |||
114 | -F configfile | ||
115 | Specifies an alternative per-user configuration file. If a | ||
116 | configuration file is given on the command line, the system-wide | ||
117 | configuration file (/etc/ssh/ssh_config) will be ignored. The | ||
118 | default for the per-user configuration file is ~/.ssh/config. | ||
119 | |||
120 | -f Requests ssh to go to background just before command execution. | ||
121 | This is useful if ssh is going to ask for passwords or | ||
122 | passphrases, but the user wants it in the background. This | ||
123 | implies -n. The recommended way to start X11 programs at a | ||
124 | remote site is with something like ssh -f host xterm. | ||
125 | |||
126 | If the ExitOnForwardFailure configuration option is set to | ||
127 | ``yes'', then a client started with -f will wait for all remote | ||
128 | port forwards to be successfully established before placing | ||
129 | itself in the background. | ||
130 | |||
131 | -g Allows remote hosts to connect to local forwarded ports. | ||
132 | |||
133 | -I pkcs11 | ||
134 | Specify the PKCS#11 shared library ssh should use to communicate | ||
135 | with a PKCS#11 token providing the user's private RSA key. | ||
136 | |||
137 | -i identity_file | ||
138 | Selects a file from which the identity (private key) for public | ||
139 | key authentication is read. The default is ~/.ssh/identity for | ||
140 | protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa and | ||
141 | ~/.ssh/id_rsa for protocol version 2. Identity files may also be | ||
142 | specified on a per-host basis in the configuration file. It is | ||
143 | possible to have multiple -i options (and multiple identities | ||
144 | specified in configuration files). ssh will also try to load | ||
145 | certificate information from the filename obtained by appending | ||
146 | -cert.pub to identity filenames. | ||
147 | |||
148 | -K Enables GSSAPI-based authentication and forwarding (delegation) | ||
149 | of GSSAPI credentials to the server. | ||
150 | |||
151 | -k Disables forwarding (delegation) of GSSAPI credentials to the | ||
152 | server. | ||
153 | |||
154 | -L [bind_address:]port:host:hostport | ||
155 | Specifies that the given port on the local (client) host is to be | ||
156 | forwarded to the given host and port on the remote side. This | ||
157 | works by allocating a socket to listen to port on the local side, | ||
158 | optionally bound to the specified bind_address. Whenever a | ||
159 | connection is made to this port, the connection is forwarded over | ||
160 | the secure channel, and a connection is made to host port | ||
161 | hostport from the remote machine. Port forwardings can also be | ||
162 | specified in the configuration file. IPv6 addresses can be | ||
163 | specified 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. | ||
166 | However, an explicit bind_address may be used to bind the | ||
167 | connection to a specific address. The bind_address of | ||
168 | ``localhost'' indicates that the listening port be bound for | ||
169 | local use only, while an empty address or `*' indicates that the | ||
170 | port should be available 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 | ||
177 | sharing. Multiple -M options places ssh into ``master'' mode | ||
178 | with confirmation required before slave connections are accepted. | ||
179 | Refer to the description of ControlMaster in ssh_config(5) for | ||
180 | details. | ||
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 | ||
188 | forwarding 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 | ||
193 | machine. For example, ssh -n shadows.cs.hut.fi emacs & will | ||
194 | start an emacs on shadows.cs.hut.fi, and the X11 connection will | ||
195 | be automatically forwarded over an encrypted channel. The ssh | ||
196 | program will be put in the background. (This does not work if | ||
197 | ssh needs to ask for a password or passphrase; see also the -f | ||
198 | option.) | ||
199 | |||
200 | -O ctl_cmd | ||
201 | Control an active connection multiplexing master process. When | ||
202 | the -O option is specified, the ctl_cmd argument is interpreted | ||
203 | and passed to the master process. Valid commands are: ``check'' | ||
204 | (check that the master process is running), ``forward'' (request | ||
205 | forwardings without command execution), ``exit'' (request the | ||
206 | master to exit), and ``stop'' (request the master to stop | ||
207 | accepting further multiplexing requests). | ||
208 | |||
209 | -o option | ||
210 | Can be used to give options in the format used in the | ||
211 | configuration file. This is useful for specifying options for | ||
212 | which there is no separate command-line flag. For full details | ||
213 | of the options listed below, and their possible values, see | ||
214 | ssh_config(5). | ||
215 | |||
216 | AddressFamily | ||
217 | BatchMode | ||
218 | BindAddress | ||
219 | ChallengeResponseAuthentication | ||
220 | CheckHostIP | ||
221 | Cipher | ||
222 | Ciphers | ||
223 | ClearAllForwardings | ||
224 | Compression | ||
225 | CompressionLevel | ||
226 | ConnectionAttempts | ||
227 | ConnectTimeout | ||
228 | ControlMaster | ||
229 | ControlPath | ||
230 | DynamicForward | ||
231 | EscapeChar | ||
232 | ExitOnForwardFailure | ||
233 | ForwardAgent | ||
234 | ForwardX11 | ||
235 | ForwardX11Trusted | ||
236 | GatewayPorts | ||
237 | GlobalKnownHostsFile | ||
238 | GSSAPIAuthentication | ||
239 | GSSAPIDelegateCredentials | ||
240 | HashKnownHosts | ||
241 | Host | ||
242 | HostbasedAuthentication | ||
243 | HostKeyAlgorithms | ||
244 | HostKeyAlias | ||
245 | HostName | ||
246 | IdentityFile | ||
247 | IdentitiesOnly | ||
248 | IPQoS | ||
249 | KbdInteractiveDevices | ||
250 | KexAlgorithms | ||
251 | LocalCommand | ||
252 | LocalForward | ||
253 | LogLevel | ||
254 | MACs | ||
255 | NoHostAuthenticationForLocalhost | ||
256 | NumberOfPasswordPrompts | ||
257 | PasswordAuthentication | ||
258 | PermitLocalCommand | ||
259 | PKCS11Provider | ||
260 | Port | ||
261 | PreferredAuthentications | ||
262 | Protocol | ||
263 | ProxyCommand | ||
264 | PubkeyAuthentication | ||
265 | RekeyLimit | ||
266 | RemoteForward | ||
267 | RequestTTY | ||
268 | RhostsRSAAuthentication | ||
269 | RSAAuthentication | ||
270 | SendEnv | ||
271 | ServerAliveInterval | ||
272 | ServerAliveCountMax | ||
273 | StrictHostKeyChecking | ||
274 | TCPKeepAlive | ||
275 | Tunnel | ||
276 | TunnelDevice | ||
277 | UsePrivilegedPort | ||
278 | User | ||
279 | UserKnownHostsFile | ||
280 | VerifyHostKeyDNS | ||
281 | VisualHostKey | ||
282 | XAuthLocation | ||
283 | |||
284 | -p port | ||
285 | Port to connect to on the remote host. This can be specified on | ||
286 | a per-host basis in the configuration file. | ||
287 | |||
288 | -q Quiet mode. Causes most warning and diagnostic messages to be | ||
289 | suppressed. | ||
290 | |||
291 | -R [bind_address:]port:host:hostport | ||
292 | Specifies that the given port on the remote (server) host is to | ||
293 | be forwarded to the given host and port on the local side. This | ||
294 | works by allocating a socket to listen to port on the remote | ||
295 | side, and whenever a connection is made to this port, the | ||
296 | connection is forwarded over the secure channel, and a connection | ||
297 | is made to host port hostport from the local machine. | ||
298 | |||
299 | Port forwardings can also be specified in the configuration file. | ||
300 | Privileged ports can be forwarded only when logging in as root on | ||
301 | the remote machine. IPv6 addresses can be specified by enclosing | ||
302 | the address in square braces. | ||
303 | |||
304 | By default, the listening socket on the server will be bound to | ||
305 | the loopback interface only. This may be overridden by | ||
306 | specifying a bind_address. An empty bind_address, or the address | ||
307 | `*', indicates that the remote socket should listen on all | ||
308 | interfaces. Specifying a remote bind_address will only succeed | ||
309 | if the server's GatewayPorts option is enabled (see | ||
310 | sshd_config(5)). | ||
311 | |||
312 | If the port argument is `0', the listen port will be dynamically | ||
313 | allocated on the server and reported to the client at run time. | ||
314 | When used together with -O forward the allocated port will be | ||
315 | printed to the standard output. | ||
316 | |||
317 | -S ctl_path | ||
318 | Specifies the location of a control socket for connection | ||
319 | sharing, or the string ``none'' to disable connection sharing. | ||
320 | Refer to the description of ControlPath and ControlMaster in | ||
321 | ssh_config(5) for details. | ||
322 | |||
323 | -s May be used to request invocation of a subsystem on the remote | ||
324 | system. Subsystems are a feature of the SSH2 protocol which | ||
325 | facilitate the use of SSH as a secure transport for other | ||
326 | applications (eg. sftp(1)). The subsystem is specified as the | ||
327 | remote command. | ||
328 | |||
329 | -T Disable pseudo-tty allocation. | ||
330 | |||
331 | -t Force pseudo-tty allocation. This can be used to execute | ||
332 | arbitrary screen-based programs on a remote machine, which can be | ||
333 | very useful, e.g. when implementing menu services. Multiple -t | ||
334 | options force tty allocation, even if ssh has no local tty. | ||
335 | |||
336 | -V Display the version number and exit. | ||
337 | |||
338 | -v Verbose mode. Causes ssh to print debugging messages about its | ||
339 | progress. This is helpful in debugging connection, | ||
340 | authentication, and configuration problems. Multiple -v options | ||
341 | increase the verbosity. The maximum is 3. | ||
342 | |||
343 | -W host:port | ||
344 | Requests that standard input and output on the client be | ||
345 | forwarded to host on port over the secure channel. Implies -N, | ||
346 | -T, ExitOnForwardFailure and ClearAllForwardings and works with | ||
347 | Protocol version 2 only. | ||
348 | |||
349 | -w local_tun[:remote_tun] | ||
350 | Requests tunnel device forwarding with the specified tun(4) | ||
351 | devices between the client (local_tun) and the server | ||
352 | (remote_tun). | ||
353 | |||
354 | The devices may be specified by numerical ID or the keyword | ||
355 | ``any'', which uses the next available tunnel device. If | ||
356 | remote_tun is not specified, it defaults to ``any''. See also | ||
357 | the Tunnel and TunnelDevice directives in ssh_config(5). If the | ||
358 | Tunnel directive is unset, it is set to the default tunnel mode, | ||
359 | which is ``point-to-point''. | ||
360 | |||
361 | -X Enables X11 forwarding. This can also be specified on a per-host | ||
362 | basis in a configuration file. | ||
363 | |||
364 | X11 forwarding should be enabled with caution. Users with the | ||
365 | ability to bypass file permissions on the remote host (for the | ||
366 | user's X authorization database) can access the local X11 display | ||
367 | through the forwarded connection. An attacker may then be able | ||
368 | to perform activities such as keystroke monitoring. | ||
369 | |||
370 | For this reason, X11 forwarding is subjected to X11 SECURITY | ||
371 | extension restrictions by default. Please refer to the ssh -Y | ||
372 | option and the ForwardX11Trusted directive in ssh_config(5) for | ||
373 | more information. | ||
374 | |||
375 | -x Disables X11 forwarding. | ||
376 | |||
377 | -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not | ||
378 | subjected to the X11 SECURITY extension controls. | ||
379 | |||
380 | -y Send log information using the syslog(3) system module. By | ||
381 | default this information is sent to stderr. | ||
382 | |||
383 | ssh may additionally obtain configuration data from a per-user | ||
384 | configuration file and a system-wide configuration file. The file format | ||
385 | and configuration options are described in ssh_config(5). | ||
386 | |||
387 | AUTHENTICATION | ||
388 | The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to | ||
389 | use protocol 2 only, though this can be changed via the Protocol option | ||
390 | in ssh_config(5) or the -1 and -2 options (see above). Both protocols | ||
391 | support similar authentication methods, but protocol 2 is the default | ||
392 | since it provides additional mechanisms for confidentiality (the traffic | ||
393 | is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and | ||
394 | integrity (hmac-md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512, umac-64, | ||
395 | hmac-ripemd160). Protocol 1 lacks a strong mechanism for ensuring the | ||
396 | integrity of the connection. | ||
397 | |||
398 | The methods available for authentication are: GSSAPI-based | ||
399 | authentication, host-based authentication, public key authentication, | ||
400 | challenge-response authentication, and password authentication. | ||
401 | Authentication methods are tried in the order specified above, though | ||
402 | protocol 2 has a configuration option to change the default order: | ||
403 | PreferredAuthentications. | ||
404 | |||
405 | Host-based authentication works as follows: If the machine the user logs | ||
406 | in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote | ||
407 | machine, and the user names are the same on both sides, or if the files | ||
408 | ~/.rhosts or ~/.shosts exist in the user's home directory on the remote | ||
409 | machine and contain a line containing the name of the client machine and | ||
410 | the name of the user on that machine, the user is considered for login. | ||
411 | Additionally, the server must be able to verify the client's host key | ||
412 | (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts, | ||
413 | below) for login to be permitted. This authentication method closes | ||
414 | security holes due to IP spoofing, DNS spoofing, and routing spoofing. | ||
415 | [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the | ||
416 | rlogin/rsh protocol in general, are inherently insecure and should be | ||
417 | disabled if security is desired.] | ||
418 | |||
419 | Public key authentication works as follows: The scheme is based on | ||
420 | public-key cryptography, using cryptosystems where encryption and | ||
421 | decryption are done using separate keys, and it is unfeasible to derive | ||
422 | the decryption key from the encryption key. The idea is that each user | ||
423 | creates a public/private key pair for authentication purposes. The | ||
424 | server knows the public key, and only the user knows the private key. | ||
425 | ssh implements public key authentication protocol automatically, using | ||
426 | one of the DSA, ECDSA or RSA algorithms. Protocol 1 is restricted to | ||
427 | using only RSA keys, but protocol 2 may use any. The HISTORY section of | ||
428 | ssl(8) contains a brief discussion of the DSA and RSA algorithms. | ||
429 | |||
430 | The file ~/.ssh/authorized_keys lists the public keys that are permitted | ||
431 | for logging in. When the user logs in, the ssh program tells the server | ||
432 | which key pair it would like to use for authentication. The client | ||
433 | proves that it has access to the private key and the server checks that | ||
434 | the corresponding public key is authorized to accept the account. | ||
435 | |||
436 | The user creates his/her key pair by running ssh-keygen(1). This stores | ||
437 | the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol | ||
438 | 2 DSA), ~/.ssh/id_ecdsa (protocol 2 ECDSA), or ~/.ssh/id_rsa (protocol 2 | ||
439 | RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1), | ||
440 | ~/.ssh/id_dsa.pub (protocol 2 DSA), ~/.ssh/id_ecdsa.pub (protocol 2 | ||
441 | ECDSA), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home | ||
442 | directory. The user should then copy the public key to | ||
443 | ~/.ssh/authorized_keys in his/her home directory on the remote machine. | ||
444 | The authorized_keys file corresponds to the conventional ~/.rhosts file, | ||
445 | and has one key per line, though the lines can be very long. After this, | ||
446 | the user can log in without giving the password. | ||
447 | |||
448 | A variation on public key authentication is available in the form of | ||
449 | certificate authentication: instead of a set of public/private keys, | ||
450 | signed certificates are used. This has the advantage that a single | ||
451 | trusted certification authority can be used in place of many | ||
452 | public/private keys. See the CERTIFICATES section of ssh-keygen(1) for | ||
453 | more information. | ||
454 | |||
455 | The most convenient way to use public key or certificate authentication | ||
456 | may be with an authentication agent. See ssh-agent(1) for more | ||
457 | information. | ||
458 | |||
459 | Challenge-response authentication works as follows: The server sends an | ||
460 | arbitrary "challenge" text, and prompts for a response. Protocol 2 | ||
461 | allows multiple challenges and responses; protocol 1 is restricted to | ||
462 | just one challenge/response. Examples of challenge-response | ||
463 | authentication include BSD Authentication (see login.conf(5)) and PAM | ||
464 | (some non-OpenBSD systems). | ||
465 | |||
466 | Finally, if other authentication methods fail, ssh prompts the user for a | ||
467 | password. The password is sent to the remote host for checking; however, | ||
468 | since all communications are encrypted, the password cannot be seen by | ||
469 | someone listening on the network. | ||
470 | |||
471 | ssh automatically maintains and checks a database containing | ||
472 | identification for all hosts it has ever been used with. Host keys are | ||
473 | stored in ~/.ssh/known_hosts in the user's home directory. Additionally, | ||
474 | the file /etc/ssh/ssh_known_hosts is automatically checked for known | ||
475 | hosts. Any new hosts are automatically added to the user's file. If a | ||
476 | host's identification ever changes, ssh warns about this and disables | ||
477 | password authentication to prevent server spoofing or man-in-the-middle | ||
478 | attacks, which could otherwise be used to circumvent the encryption. The | ||
479 | StrictHostKeyChecking option can be used to control logins to machines | ||
480 | whose host key is not known or has changed. | ||
481 | |||
482 | When the user's identity has been accepted by the server, the server | ||
483 | either executes the given command, or logs into the machine and gives the | ||
484 | user a normal shell on the remote machine. All communication with the | ||
485 | remote command or shell will be automatically encrypted. | ||
486 | |||
487 | If a pseudo-terminal has been allocated (normal login session), the user | ||
488 | may use the escape characters noted below. | ||
489 | |||
490 | If no pseudo-tty has been allocated, the session is transparent and can | ||
491 | be used to reliably transfer binary data. On most systems, setting the | ||
492 | escape character to ``none'' will also make the session transparent even | ||
493 | if a tty is used. | ||
494 | |||
495 | The session terminates when the command or shell on the remote machine | ||
496 | exits and all X11 and TCP connections have been closed. | ||
497 | |||
498 | ESCAPE CHARACTERS | ||
499 | When a pseudo-terminal has been requested, ssh supports a number of | ||
500 | functions through the use of an escape character. | ||
501 | |||
502 | A single tilde character can be sent as ~~ or by following the tilde by a | ||
503 | character other than those described below. The escape character must | ||
504 | always follow a newline to be interpreted as special. The escape | ||
505 | character can be changed in configuration files using the EscapeChar | ||
506 | configuration directive or on the command line by the -e option. | ||
507 | |||
508 | The supported escapes (assuming the default `~') are: | ||
509 | |||
510 | ~. Disconnect. | ||
511 | |||
512 | ~^Z Background ssh. | ||
513 | |||
514 | ~# List forwarded connections. | ||
515 | |||
516 | ~& Background ssh at logout when waiting for forwarded connection / | ||
517 | X11 sessions to terminate. | ||
518 | |||
519 | ~? Display a list of escape characters. | ||
520 | |||
521 | ~B Send a BREAK to the remote system (only useful for SSH protocol | ||
522 | version 2 and if the peer supports it). | ||
523 | |||
524 | ~C Open command line. Currently this allows the addition of port | ||
525 | forwardings using the -L, -R and -D options (see above). It also | ||
526 | allows the cancellation of existing remote port-forwardings using | ||
527 | -KR[bind_address:]port. !command allows the user to execute a | ||
528 | local command if the PermitLocalCommand option is enabled in | ||
529 | ssh_config(5). Basic help is available, using the -h option. | ||
530 | |||
531 | ~R Request rekeying of the connection (only useful for SSH protocol | ||
532 | version 2 and if the peer supports it). | ||
533 | |||
534 | TCP FORWARDING | ||
535 | Forwarding of arbitrary TCP connections over the secure channel can be | ||
536 | specified either on the command line or in a configuration file. One | ||
537 | possible application of TCP forwarding is a secure connection to a mail | ||
538 | server; another is going through firewalls. | ||
539 | |||
540 | In the example below, we look at encrypting communication between an IRC | ||
541 | client and server, even though the IRC server does not directly support | ||
542 | encrypted communications. This works as follows: the user connects to | ||
543 | the remote host using ssh, specifying a port to be used to forward | ||
544 | connections to the remote server. After that it is possible to start the | ||
545 | service which is to be encrypted on the client machine, connecting to the | ||
546 | same local port, and ssh will encrypt and forward the connection. | ||
547 | |||
548 | The following example tunnels an IRC session from client machine | ||
549 | ``127.0.0.1'' (localhost) to remote server ``server.example.com'': | ||
550 | |||
551 | $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10 | ||
552 | $ irc -c '#users' -p 1234 pinky 127.0.0.1 | ||
553 | |||
554 | This tunnels a connection to IRC server ``server.example.com'', joining | ||
555 | channel ``#users'', nickname ``pinky'', using port 1234. It doesn't | ||
556 | matter which port is used, as long as it's greater than 1023 (remember, | ||
557 | only root can open sockets on privileged ports) and doesn't conflict with | ||
558 | any ports already in use. The connection is forwarded to port 6667 on | ||
559 | the remote server, since that's the standard port for IRC services. | ||
560 | |||
561 | The -f option backgrounds ssh and the remote command ``sleep 10'' is | ||
562 | specified to allow an amount of time (10 seconds, in the example) to | ||
563 | start the service which is to be tunnelled. If no connections are made | ||
564 | within the time specified, ssh will exit. | ||
565 | |||
566 | X11 FORWARDING | ||
567 | If the ForwardX11 variable is set to ``yes'' (or see the description of | ||
568 | the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY | ||
569 | environment variable is set), the connection to the X11 display is | ||
570 | automatically forwarded to the remote side in such a way that any X11 | ||
571 | programs started from the shell (or command) will go through the | ||
572 | encrypted channel, and the connection to the real X server will be made | ||
573 | from the local machine. The user should not manually set DISPLAY. | ||
574 | Forwarding of X11 connections can be configured on the command line or in | ||
575 | configuration files. | ||
576 | |||
577 | The DISPLAY value set by ssh will point to the server machine, but with a | ||
578 | display number greater than zero. This is normal, and happens because | ||
579 | ssh creates a ``proxy'' X server on the server machine for forwarding the | ||
580 | connections over the encrypted channel. | ||
581 | |||
582 | ssh will also automatically set up Xauthority data on the server machine. | ||
583 | For this purpose, it will generate a random authorization cookie, store | ||
584 | it in Xauthority on the server, and verify that any forwarded connections | ||
585 | carry this cookie and replace it by the real cookie when the connection | ||
586 | is opened. The real authentication cookie is never sent to the server | ||
587 | machine (and no cookies are sent in the plain). | ||
588 | |||
589 | If the ForwardAgent variable is set to ``yes'' (or see the description of | ||
590 | the -A and -a options above) and the user is using an authentication | ||
591 | agent, the connection to the agent is automatically forwarded to the | ||
592 | remote side. | ||
593 | |||
594 | VERIFYING HOST KEYS | ||
595 | When connecting to a server for the first time, a fingerprint of the | ||
596 | server's public key is presented to the user (unless the option | ||
597 | StrictHostKeyChecking has been disabled). Fingerprints can be determined | ||
598 | using ssh-keygen(1): | ||
599 | |||
600 | $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key | ||
601 | |||
602 | If the fingerprint is already known, it can be matched and the key can be | ||
603 | accepted or rejected. Because of the difficulty of comparing host keys | ||
604 | just by looking at hex strings, there is also support to compare host | ||
605 | keys visually, using random art. By setting the VisualHostKey option to | ||
606 | ``yes'', a small ASCII graphic gets displayed on every login to a server, | ||
607 | no matter if the session itself is interactive or not. By learning the | ||
608 | pattern a known server produces, a user can easily find out that the host | ||
609 | key has changed when a completely different pattern is displayed. | ||
610 | Because these patterns are not unambiguous however, a pattern that looks | ||
611 | similar to the pattern remembered only gives a good probability that the | ||
612 | host key is the same, not guaranteed proof. | ||
613 | |||
614 | To get a listing of the fingerprints along with their random art for all | ||
615 | known hosts, the following command line can be used: | ||
616 | |||
617 | $ ssh-keygen -lv -f ~/.ssh/known_hosts | ||
618 | |||
619 | If the fingerprint is unknown, an alternative method of verification is | ||
620 | available: SSH fingerprints verified by DNS. An additional resource | ||
621 | record (RR), SSHFP, is added to a zonefile and the connecting client is | ||
622 | able to match the fingerprint with that of the key presented. | ||
623 | |||
624 | In this example, we are connecting a client to a server, | ||
625 | ``host.example.com''. The SSHFP resource records should first be added | ||
626 | to the zonefile for host.example.com: | ||
627 | |||
628 | $ ssh-keygen -r host.example.com. | ||
629 | |||
630 | The output lines will have to be added to the zonefile. To check that | ||
631 | the zone is answering fingerprint queries: | ||
632 | |||
633 | $ dig -t SSHFP host.example.com | ||
634 | |||
635 | Finally the client connects: | ||
636 | |||
637 | $ ssh -o "VerifyHostKeyDNS ask" host.example.com | ||
638 | [...] | ||
639 | Matching host key fingerprint found in DNS. | ||
640 | Are you sure you want to continue connecting (yes/no)? | ||
641 | |||
642 | See the VerifyHostKeyDNS option in ssh_config(5) for more information. | ||
643 | |||
644 | SSH-BASED VIRTUAL PRIVATE NETWORKS | ||
645 | ssh contains support for Virtual Private Network (VPN) tunnelling using | ||
646 | the tun(4) network pseudo-device, allowing two networks to be joined | ||
647 | securely. The sshd_config(5) configuration option PermitTunnel controls | ||
648 | whether the server supports this, and at what level (layer 2 or 3 | ||
649 | traffic). | ||
650 | |||
651 | The following example would connect client network 10.0.50.0/24 with | ||
652 | remote network 10.0.99.0/24 using a point-to-point connection from | ||
653 | 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway | ||
654 | to the remote network, at 192.168.1.15, allows it. | ||
655 | |||
656 | On the client: | ||
657 | |||
658 | # ssh -f -w 0:1 192.168.1.15 true | ||
659 | # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252 | ||
660 | # route add 10.0.99.0/24 10.1.1.2 | ||
661 | |||
662 | On the server: | ||
663 | |||
664 | # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252 | ||
665 | # route add 10.0.50.0/24 10.1.1.1 | ||
666 | |||
667 | Client access may be more finely tuned via the /root/.ssh/authorized_keys | ||
668 | file (see below) and the PermitRootLogin server option. The following | ||
669 | entry would permit connections on tun(4) device 1 from user ``jane'' and | ||
670 | on tun device 2 from user ``john'', if PermitRootLogin is set to | ||
671 | ``forced-commands-only'': | ||
672 | |||
673 | tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane | ||
674 | tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john | ||
675 | |||
676 | Since an SSH-based setup entails a fair amount of overhead, it may be | ||
677 | more suited to temporary setups, such as for wireless VPNs. More | ||
678 | permanent VPNs are better provided by tools such as ipsecctl(8) and | ||
679 | isakmpd(8). | ||
680 | |||
681 | ENVIRONMENT | ||
682 | ssh will normally set the following environment variables: | ||
683 | |||
684 | DISPLAY The DISPLAY variable indicates the location of the | ||
685 | X11 server. It is automatically set by ssh to | ||
686 | point to a value of the form ``hostname:n'', where | ||
687 | ``hostname'' indicates the host where the shell | ||
688 | runs, and `n' is an integer >= 1. ssh uses this | ||
689 | special value to forward X11 connections over the | ||
690 | secure channel. The user should normally not set | ||
691 | DISPLAY explicitly, as that will render the X11 | ||
692 | connection insecure (and will require the user to | ||
693 | manually copy any required authorization cookies). | ||
694 | |||
695 | HOME Set to the path of the user's home directory. | ||
696 | |||
697 | LOGNAME Synonym for USER; set for compatibility with | ||
698 | systems that use this variable. | ||
699 | |||
700 | MAIL Set to the path of the user's mailbox. | ||
701 | |||
702 | PATH Set to the default PATH, as specified when | ||
703 | compiling ssh. | ||
704 | |||
705 | SSH_ASKPASS If ssh needs a passphrase, it will read the | ||
706 | passphrase from the current terminal if it was run | ||
707 | from a terminal. If ssh does not have a terminal | ||
708 | associated with it but DISPLAY and SSH_ASKPASS are | ||
709 | set, it will execute the program specified by | ||
710 | SSH_ASKPASS and open an X11 window to read the | ||
711 | passphrase. This is particularly useful when | ||
712 | calling ssh from a .xsession or related script. | ||
713 | (Note that on some machines it may be necessary to | ||
714 | redirect the input from /dev/null to make this | ||
715 | work.) | ||
716 | |||
717 | SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to | ||
718 | communicate with the agent. | ||
719 | |||
720 | SSH_CONNECTION Identifies the client and server ends of the | ||
721 | connection. The variable contains four space- | ||
722 | separated values: client IP address, client port | ||
723 | number, server IP address, and server port number. | ||
724 | |||
725 | SSH_ORIGINAL_COMMAND This variable contains the original command line if | ||
726 | a forced command is executed. It can be used to | ||
727 | extract the original arguments. | ||
728 | |||
729 | SSH_TTY This is set to the name of the tty (path to the | ||
730 | device) associated with the current shell or | ||
731 | command. If the current session has no tty, this | ||
732 | variable is not set. | ||
733 | |||
734 | TZ This variable is set to indicate the present time | ||
735 | zone if it was set when the daemon was started | ||
736 | (i.e. the daemon passes the value on to new | ||
737 | connections). | ||
738 | |||
739 | USER Set to the name of the user logging in. | ||
740 | |||
741 | Additionally, ssh reads ~/.ssh/environment, and adds lines of the format | ||
742 | ``VARNAME=value'' to the environment if the file exists and users are | ||
743 | allowed to change their environment. For more information, see the | ||
744 | PermitUserEnvironment option in sshd_config(5). | ||
745 | |||
746 | FILES | ||
747 | ~/.rhosts | ||
748 | This file is used for host-based authentication (see above). On | ||
749 | some machines this file may need to be world-readable if the | ||
750 | user's home directory is on an NFS partition, because sshd(8) | ||
751 | reads it as root. Additionally, this file must be owned by the | ||
752 | user, and must not have write permissions for anyone else. The | ||
753 | recommended permission for most machines is read/write for the | ||
754 | user, and not accessible by others. | ||
755 | |||
756 | ~/.shosts | ||
757 | This file is used in exactly the same way as .rhosts, but allows | ||
758 | host-based authentication without permitting login with | ||
759 | rlogin/rsh. | ||
760 | |||
761 | ~/.ssh/ | ||
762 | This directory is the default location for all user-specific | ||
763 | configuration and authentication information. There is no | ||
764 | general requirement to keep the entire contents of this directory | ||
765 | secret, but the recommended permissions are read/write/execute | ||
766 | for the user, and not accessible by others. | ||
767 | |||
768 | ~/.ssh/authorized_keys | ||
769 | Lists the public keys (DSA/ECDSA/RSA) that can be used for | ||
770 | logging in as this user. The format of this file is described in | ||
771 | the sshd(8) manual page. This file is not highly sensitive, but | ||
772 | the recommended permissions are read/write for the user, and not | ||
773 | accessible by others. | ||
774 | |||
775 | ~/.ssh/config | ||
776 | This is the per-user configuration file. The file format and | ||
777 | configuration options are described in ssh_config(5). Because of | ||
778 | the potential for abuse, this file must have strict permissions: | ||
779 | read/write for the user, and not accessible by others. | ||
780 | |||
781 | ~/.ssh/environment | ||
782 | Contains additional definitions for environment variables; see | ||
783 | ENVIRONMENT, above. | ||
784 | |||
785 | ~/.ssh/identity | ||
786 | ~/.ssh/id_dsa | ||
787 | ~/.ssh/id_ecdsa | ||
788 | ~/.ssh/id_rsa | ||
789 | Contains the private key for authentication. These files contain | ||
790 | sensitive data and should be readable by the user but not | ||
791 | accessible by others (read/write/execute). ssh will simply | ||
792 | ignore a private key file if it is accessible by others. It is | ||
793 | possible to specify a passphrase when generating the key which | ||
794 | will be used to encrypt the sensitive part of this file using | ||
795 | 3DES. | ||
796 | |||
797 | ~/.ssh/identity.pub | ||
798 | ~/.ssh/id_dsa.pub | ||
799 | ~/.ssh/id_ecdsa.pub | ||
800 | ~/.ssh/id_rsa.pub | ||
801 | Contains the public key for authentication. These files are not | ||
802 | sensitive and can (but need not) be readable by anyone. | ||
803 | |||
804 | ~/.ssh/known_hosts | ||
805 | Contains a list of host keys for all hosts the user has logged | ||
806 | into that are not already in the systemwide list of known host | ||
807 | keys. See sshd(8) for further details of the format of this | ||
808 | file. | ||
809 | |||
810 | ~/.ssh/rc | ||
811 | Commands in this file are executed by ssh when the user logs in, | ||
812 | just before the user's shell (or command) is started. See the | ||
813 | sshd(8) manual page for more information. | ||
814 | |||
815 | /etc/hosts.equiv | ||
816 | This file is for host-based authentication (see above). It | ||
817 | should only be writable by root. | ||
818 | |||
819 | /etc/shosts.equiv | ||
820 | This file is used in exactly the same way as hosts.equiv, but | ||
821 | allows host-based authentication without permitting login with | ||
822 | rlogin/rsh. | ||
823 | |||
824 | /etc/ssh/ssh_config | ||
825 | Systemwide configuration file. The file format and configuration | ||
826 | options are described in ssh_config(5). | ||
827 | |||
828 | /etc/ssh/ssh_host_key | ||
829 | /etc/ssh/ssh_host_dsa_key | ||
830 | /etc/ssh/ssh_host_ecdsa_key | ||
831 | /etc/ssh/ssh_host_rsa_key | ||
832 | These three files contain the private parts of the host keys and | ||
833 | are used for host-based authentication. If protocol version 1 is | ||
834 | used, ssh must be setuid root, since the host key is readable | ||
835 | only by root. For protocol version 2, ssh uses ssh-keysign(8) to | ||
836 | access the host keys, eliminating the requirement that ssh be | ||
837 | setuid root when host-based authentication is used. By default | ||
838 | ssh is not setuid root. | ||
839 | |||
840 | /etc/ssh/ssh_known_hosts | ||
841 | Systemwide list of known host keys. This file should be prepared | ||
842 | by the system administrator to contain the public host keys of | ||
843 | all machines in the organization. It should be world-readable. | ||
844 | See sshd(8) for further details of the format of this file. | ||
845 | |||
846 | /etc/ssh/sshrc | ||
847 | Commands in this file are executed by ssh when the user logs in, | ||
848 | just before the user's shell (or command) is started. See the | ||
849 | sshd(8) manual page for more information. | ||
850 | |||
851 | EXIT STATUS | ||
852 | ssh exits with the exit status of the remote command or with 255 if an | ||
853 | error occurred. | ||
854 | |||
855 | SEE ALSO | ||
856 | scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1), | ||
857 | tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8) | ||
858 | |||
859 | The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006. | ||
860 | |||
861 | The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006. | ||
862 | |||
863 | The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006. | ||
864 | |||
865 | The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006. | ||
866 | |||
867 | The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006. | ||
868 | |||
869 | Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC | ||
870 | 4255, 2006. | ||
871 | |||
872 | Generic Message Exchange Authentication for the Secure Shell Protocol | ||
873 | (SSH), RFC 4256, 2006. | ||
874 | |||
875 | The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006. | ||
876 | |||
877 | The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006. | ||
878 | |||
879 | Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer | ||
880 | Protocol, RFC 4345, 2006. | ||
881 | |||
882 | Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer | ||
883 | Protocol, RFC 4419, 2006. | ||
884 | |||
885 | The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006. | ||
886 | |||
887 | Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer, | ||
888 | RFC 5656, 2009. | ||
889 | |||
890 | A. Perrig and D. Song, Hash Visualization: a New Technique to improve | ||
891 | Real-World Security, 1999, International Workshop on Cryptographic | ||
892 | Techniques and E-Commerce (CrypTEC '99). | ||
893 | |||
894 | AUTHORS | ||
895 | OpenSSH is a derivative of the original and free ssh 1.2.12 release by | ||
896 | Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo | ||
897 | de Raadt and Dug Song removed many bugs, re-added newer features and | ||
898 | created OpenSSH. Markus Friedl contributed the support for SSH protocol | ||
899 | versions 1.5 and 2.0. | ||
900 | |||
901 | OpenBSD 5.0 August 2, 2011 OpenBSD 5.0 | ||