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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 [-1246AaCfgkNnqsTtVvXxY] [-b bind_address] [-c cipher_spec] [-D port] | ||
8 | [-e escape_char] [-F configfile] [-i identity_file] | ||
9 | [-L port:host:hostport] [-l login_name] [-m mac_spec] [-o option] | ||
10 | [-p port] [-R port:host:hostport] [user@]hostname [command] | ||
11 | |||
12 | DESCRIPTION | ||
13 | ssh (SSH client) is a program for logging into a remote machine and for | ||
14 | executing commands on a remote machine. It is intended to replace rlogin | ||
15 | and rsh, and provide secure encrypted communications between two untrust- | ||
16 | ed hosts over an insecure network. X11 connections and arbitrary TCP/IP | ||
17 | ports can also be forwarded over the secure channel. | ||
18 | |||
19 | ssh connects and logs into the specified hostname (with optional user | ||
20 | name). The user must prove his/her identity to the remote machine using | ||
21 | one of several methods depending on the protocol version used. | ||
22 | |||
23 | If command is specified, command is executed on the remote host instead | ||
24 | of a login shell. | ||
25 | |||
26 | SSH protocol version 1 | ||
27 | First, if the machine the user logs in from is listed in /etc/hosts.equiv | ||
28 | or /etc/shosts.equiv on the remote machine, and the user names are the | ||
29 | same on both sides, the user is immediately permitted to log in. Second, | ||
30 | if .rhosts or .shosts exists in the user's home directory on the remote | ||
31 | machine and contains a line containing the name of the client machine and | ||
32 | the name of the user on that machine, the user is permitted to log in. | ||
33 | This form of authentication alone is normally not allowed by the server | ||
34 | because it is not secure. | ||
35 | |||
36 | The second authentication method is the rhosts or hosts.equiv method com- | ||
37 | bined with RSA-based host authentication. It means that if the login | ||
38 | would be permitted by $HOME/.rhosts, $HOME/.shosts, /etc/hosts.equiv, or | ||
39 | /etc/shosts.equiv, and if additionally the server can verify the client's | ||
40 | host key (see /etc/ssh/ssh_known_hosts and $HOME/.ssh/known_hosts in the | ||
41 | FILES section), only then is login permitted. This authentication method | ||
42 | closes security holes due to IP spoofing, DNS spoofing and routing spoof- | ||
43 | ing. [Note to the administrator: /etc/hosts.equiv, $HOME/.rhosts, and | ||
44 | the rlogin/rsh protocol in general, are inherently insecure and should be | ||
45 | disabled if security is desired.] | ||
46 | |||
47 | As a third authentication method, ssh supports RSA based authentication. | ||
48 | The scheme is based on public-key cryptography: there are cryptosystems | ||
49 | where encryption and decryption are done using separate keys, and it is | ||
50 | not possible to derive the decryption key from the encryption key. RSA | ||
51 | is one such system. The idea is that each user creates a public/private | ||
52 | key pair for authentication purposes. The server knows the public key, | ||
53 | and only the user knows the private key. | ||
54 | |||
55 | The file $HOME/.ssh/authorized_keys lists the public keys that are per- | ||
56 | mitted for logging in. When the user logs in, the ssh program tells the | ||
57 | server which key pair it would like to use for authentication. The serv- | ||
58 | er checks if this key is permitted, and if so, sends the user (actually | ||
59 | the ssh program running on behalf of the user) a challenge, a random num- | ||
60 | ber, encrypted by the user's public key. The challenge can only be de- | ||
61 | crypted using the proper private key. The user's client then decrypts | ||
62 | the challenge using the private key, proving that he/she knows the pri- | ||
63 | vate key but without disclosing it to the server. | ||
64 | |||
65 | ssh implements the RSA authentication protocol automatically. The user | ||
66 | creates his/her RSA key pair by running ssh-keygen(1). This stores the | ||
67 | private key in $HOME/.ssh/identity and stores the public key in | ||
68 | $HOME/.ssh/identity.pub in the user's home directory. The user should | ||
69 | then copy the identity.pub to $HOME/.ssh/authorized_keys in his/her home | ||
70 | directory on the remote machine (the authorized_keys file corresponds to | ||
71 | the conventional $HOME/.rhosts file, and has one key per line, though the | ||
72 | lines can be very long). After this, the user can log in without giving | ||
73 | the password. RSA authentication is much more secure than rhosts authen- | ||
74 | tication. | ||
75 | |||
76 | The most convenient way to use RSA authentication may be with an authen- | ||
77 | tication agent. See ssh-agent(1) for more information. | ||
78 | |||
79 | If other authentication methods fail, ssh prompts the user for a pass- | ||
80 | word. The password is sent to the remote host for checking; however, | ||
81 | since all communications are encrypted, the password cannot be seen by | ||
82 | someone listening on the network. | ||
83 | |||
84 | SSH protocol version 2 | ||
85 | When a user connects using protocol version 2, similar authentication | ||
86 | methods are available. Using the default values for | ||
87 | PreferredAuthentications, the client will try to authenticate first using | ||
88 | the hostbased method; if this method fails, public key authentication is | ||
89 | attempted, and finally if this method fails, keyboard-interactive and | ||
90 | password authentication are tried. | ||
91 | |||
92 | The public key method is similar to RSA authentication described in the | ||
93 | previous section and allows the RSA or DSA algorithm to be used: The | ||
94 | client uses his private key, $HOME/.ssh/id_dsa or $HOME/.ssh/id_rsa, to | ||
95 | sign the session identifier and sends the result to the server. The | ||
96 | server checks whether the matching public key is listed in | ||
97 | $HOME/.ssh/authorized_keys and grants access if both the key is found and | ||
98 | the signature is correct. The session identifier is derived from a | ||
99 | shared Diffie-Hellman value and is only known to the client and the serv- | ||
100 | er. | ||
101 | |||
102 | If public key authentication fails or is not available, a password can be | ||
103 | sent encrypted to the remote host to prove the user's identity. | ||
104 | |||
105 | Additionally, ssh supports hostbased or challenge response authentica- | ||
106 | tion. | ||
107 | |||
108 | Protocol 2 provides additional mechanisms for confidentiality (the traf- | ||
109 | fic is encrypted using 3DES, Blowfish, CAST128 or Arcfour) and integrity | ||
110 | (hmac-md5, hmac-sha1). Note that protocol 1 lacks a strong mechanism for | ||
111 | ensuring the integrity of the connection. | ||
112 | |||
113 | Login session and remote execution | ||
114 | When the user's identity has been accepted by the server, the server ei- | ||
115 | ther executes the given command, or logs into the machine and gives the | ||
116 | user a normal shell on the remote machine. All communication with the | ||
117 | remote command or shell will be automatically encrypted. | ||
118 | |||
119 | If a pseudo-terminal has been allocated (normal login session), the user | ||
120 | may use the escape characters noted below. | ||
121 | |||
122 | If no pseudo-tty has been allocated, the session is transparent and can | ||
123 | be used to reliably transfer binary data. On most systems, setting the | ||
124 | escape character to ``none'' will also make the session transparent even | ||
125 | if a tty is used. | ||
126 | |||
127 | The session terminates when the command or shell on the remote machine | ||
128 | exits and all X11 and TCP/IP connections have been closed. The exit sta- | ||
129 | tus of the remote program is returned as the exit status of ssh. | ||
130 | |||
131 | Escape Characters | ||
132 | When a pseudo-terminal has been requested, ssh supports a number of func- | ||
133 | tions through the use of an escape character. | ||
134 | |||
135 | A single tilde character can be sent as ~~ or by following the tilde by a | ||
136 | character other than those described below. The escape character must | ||
137 | always follow a newline to be interpreted as special. The escape charac- | ||
138 | ter can be changed in configuration files using the EscapeChar configura- | ||
139 | tion directive or on the command line by the -e option. | ||
140 | |||
141 | The supported escapes (assuming the default `~') are: | ||
142 | |||
143 | ~. Disconnect. | ||
144 | |||
145 | ~^Z Background ssh. | ||
146 | |||
147 | ~# List forwarded connections. | ||
148 | |||
149 | ~& Background ssh at logout when waiting for forwarded connection / | ||
150 | X11 sessions to terminate. | ||
151 | |||
152 | ~? Display a list of escape characters. | ||
153 | |||
154 | ~B Send a BREAK to the remote system (only useful for SSH protocol | ||
155 | version 2 and if the peer supports it). | ||
156 | |||
157 | ~C Open command line (only useful for adding port forwardings using | ||
158 | the -L and -R options). | ||
159 | |||
160 | ~R Request rekeying of the connection (only useful for SSH protocol | ||
161 | version 2 and if the peer supports it). | ||
162 | |||
163 | X11 and TCP forwarding | ||
164 | If the ForwardX11 variable is set to ``yes'' (or see the description of | ||
165 | the -X and -x options described later) and the user is using X11 (the | ||
166 | DISPLAY environment variable is set), the connection to the X11 display | ||
167 | is automatically forwarded to the remote side in such a way that any X11 | ||
168 | programs started from the shell (or command) will go through the encrypt- | ||
169 | ed channel, and the connection to the real X server will be made from the | ||
170 | local machine. The user should not manually set DISPLAY. Forwarding of | ||
171 | X11 connections can be configured on the command line or in configuration | ||
172 | files. | ||
173 | |||
174 | The DISPLAY value set by ssh will point to the server machine, but with a | ||
175 | display number greater than zero. This is normal, and happens because | ||
176 | ssh creates a ``proxy'' X server on the server machine for forwarding the | ||
177 | connections over the encrypted channel. | ||
178 | |||
179 | ssh will also automatically set up Xauthority data on the server machine. | ||
180 | For this purpose, it will generate a random authorization cookie, store | ||
181 | it in Xauthority on the server, and verify that any forwarded connections | ||
182 | carry this cookie and replace it by the real cookie when the connection | ||
183 | is opened. The real authentication cookie is never sent to the server | ||
184 | machine (and no cookies are sent in the plain). | ||
185 | |||
186 | If the ForwardAgent variable is set to ``yes'' (or see the description of | ||
187 | the -A and -a options described later) and the user is using an authenti- | ||
188 | cation agent, the connection to the agent is automatically forwarded to | ||
189 | the remote side. | ||
190 | |||
191 | Forwarding of arbitrary TCP/IP connections over the secure channel can be | ||
192 | specified either on the command line or in a configuration file. One | ||
193 | possible application of TCP/IP forwarding is a secure connection to an | ||
194 | electronic purse; another is going through firewalls. | ||
195 | |||
196 | Server authentication | ||
197 | ssh automatically maintains and checks a database containing identifica- | ||
198 | tions for all hosts it has ever been used with. Host keys are stored in | ||
199 | $HOME/.ssh/known_hosts in the user's home directory. Additionally, the | ||
200 | file /etc/ssh/ssh_known_hosts is automatically checked for known hosts. | ||
201 | Any new hosts are automatically added to the user's file. If a host's | ||
202 | identification ever changes, ssh warns about this and disables password | ||
203 | authentication to prevent a trojan horse from getting the user's pass- | ||
204 | word. Another purpose of this mechanism is to prevent man-in-the-middle | ||
205 | attacks which could otherwise be used to circumvent the encryption. The | ||
206 | StrictHostKeyChecking option can be used to prevent logins to machines | ||
207 | whose host key is not known or has changed. | ||
208 | |||
209 | The options are as follows: | ||
210 | |||
211 | -1 Forces ssh to try protocol version 1 only. | ||
212 | |||
213 | -2 Forces ssh to try protocol version 2 only. | ||
214 | |||
215 | -4 Forces ssh to use IPv4 addresses only. | ||
216 | |||
217 | -6 Forces ssh to use IPv6 addresses only. | ||
218 | |||
219 | -A Enables forwarding of the authentication agent connection. This | ||
220 | can also be specified on a per-host basis in a configuration | ||
221 | file. | ||
222 | |||
223 | Agent forwarding should be enabled with caution. Users with the | ||
224 | ability to bypass file permissions on the remote host (for the | ||
225 | agent's Unix-domain socket) can access the local agent through | ||
226 | the forwarded connection. An attacker cannot obtain key material | ||
227 | from the agent, however they can perform operations on the keys | ||
228 | that enable them to authenticate using the identities loaded into | ||
229 | the agent. | ||
230 | |||
231 | -a Disables forwarding of the authentication agent connection. | ||
232 | |||
233 | -b bind_address | ||
234 | Specify the interface to transmit from on machines with multiple | ||
235 | interfaces or aliased addresses. | ||
236 | |||
237 | -C Requests compression of all data (including stdin, stdout, | ||
238 | stderr, and data for forwarded X11 and TCP/IP connections). The | ||
239 | compression algorithm is the same used by gzip(1), and the | ||
240 | ``level'' can be controlled by the CompressionLevel option for | ||
241 | protocol version 1. Compression is desirable on modem lines and | ||
242 | other slow connections, but will only slow down things on fast | ||
243 | networks. The default value can be set on a host-by-host basis | ||
244 | in the configuration files; see the Compression option. | ||
245 | |||
246 | -c blowfish | 3des | des | ||
247 | Selects the cipher to use for encrypting the session. 3des is | ||
248 | used by default. It is believed to be secure. 3des (triple-des) | ||
249 | is an encrypt-decrypt-encrypt triple with three different keys. | ||
250 | blowfish is a fast block cipher; it appears very secure and is | ||
251 | much faster than 3des. des is only supported in the ssh client | ||
252 | for interoperability with legacy protocol 1 implementations that | ||
253 | do not support the 3des cipher. Its use is strongly discouraged | ||
254 | due to cryptographic weaknesses. | ||
255 | |||
256 | -c cipher_spec | ||
257 | Additionally, for protocol version 2 a comma-separated list of | ||
258 | ciphers can be specified in order of preference. See Ciphers for | ||
259 | more information. | ||
260 | |||
261 | -D port | ||
262 | Specifies a local ``dynamic'' application-level port forwarding. | ||
263 | This works by allocating a socket to listen to port on the local | ||
264 | side, and whenever a connection is made to this port, the connec- | ||
265 | tion is forwarded over the secure channel, and the application | ||
266 | protocol is then used to determine where to connect to from the | ||
267 | remote machine. Currently the SOCKS4 and SOCKS5 protocols are | ||
268 | supported, and ssh will act as a SOCKS server. Only root can | ||
269 | forward privileged ports. Dynamic port forwardings can also be | ||
270 | specified in the configuration file. | ||
271 | |||
272 | -e ch | ^ch | none | ||
273 | Sets the escape character for sessions with a pty (default: `~'). | ||
274 | The escape character is only recognized at the beginning of a | ||
275 | line. The escape character followed by a dot (`.') closes the | ||
276 | connection; followed by control-Z suspends the connection; and | ||
277 | followed by itself sends the escape character once. Setting the | ||
278 | character to ``none'' disables any escapes and makes the session | ||
279 | fully transparent. | ||
280 | |||
281 | -F configfile | ||
282 | Specifies an alternative per-user configuration file. If a con- | ||
283 | figuration file is given on the command line, the system-wide | ||
284 | configuration file (/etc/ssh/ssh_config) will be ignored. The | ||
285 | default for the per-user configuration file is $HOME/.ssh/config. | ||
286 | |||
287 | -f Requests ssh to go to background just before command execution. | ||
288 | This is useful if ssh is going to ask for passwords or passphras- | ||
289 | es, but the user wants it in the background. This implies -n. | ||
290 | The recommended way to start X11 programs at a remote site is | ||
291 | with something like ssh -f host xterm. | ||
292 | |||
293 | -g Allows remote hosts to connect to local forwarded ports. | ||
294 | |||
295 | -I smartcard_device | ||
296 | Specifies which smartcard device to use. The argument is the de- | ||
297 | vice ssh should use to communicate with a smartcard used for | ||
298 | storing the user's private RSA key. | ||
299 | |||
300 | -i identity_file | ||
301 | Selects a file from which the identity (private key) for RSA or | ||
302 | DSA authentication is read. The default is $HOME/.ssh/identity | ||
303 | for protocol version 1, and $HOME/.ssh/id_rsa and | ||
304 | $HOME/.ssh/id_dsa for protocol version 2. Identity files may al- | ||
305 | so be specified on a per-host basis in the configuration file. | ||
306 | It is possible to have multiple -i options (and multiple identi- | ||
307 | ties specified in configuration files). | ||
308 | |||
309 | -k Disables forwarding (delegation) of GSSAPI credentials to the | ||
310 | server. | ||
311 | |||
312 | -L port:host:hostport | ||
313 | Specifies that the given port on the local (client) host is to be | ||
314 | forwarded to the given host and port on the remote side. This | ||
315 | works by allocating a socket to listen to port on the local side, | ||
316 | and whenever a connection is made to this port, the connection is | ||
317 | forwarded over the secure channel, and a connection is made to | ||
318 | host port hostport from the remote machine. Port forwardings can | ||
319 | also be specified in the configuration file. Only root can for- | ||
320 | ward privileged ports. IPv6 addresses can be specified with an | ||
321 | alternative syntax: port/host/hostport. | ||
322 | |||
323 | -l login_name | ||
324 | Specifies the user to log in as on the remote machine. This also | ||
325 | may be specified on a per-host basis in the configuration file. | ||
326 | |||
327 | -m mac_spec | ||
328 | Additionally, for protocol version 2 a comma-separated list of | ||
329 | MAC (message authentication code) algorithms can be specified in | ||
330 | order of preference. See the MACs keyword for more information. | ||
331 | |||
332 | -N Do not execute a remote command. This is useful for just for- | ||
333 | warding ports (protocol version 2 only). | ||
334 | |||
335 | -n Redirects stdin from /dev/null (actually, prevents reading from | ||
336 | stdin). This must be used when ssh is run in the background. A | ||
337 | common trick is to use this to run X11 programs on a remote ma- | ||
338 | chine. For example, ssh -n shadows.cs.hut.fi emacs & will start | ||
339 | an emacs on shadows.cs.hut.fi, and the X11 connection will be au- | ||
340 | tomatically forwarded over an encrypted channel. The ssh program | ||
341 | will be put in the background. (This does not work if ssh needs | ||
342 | to ask for a password or passphrase; see also the -f option.) | ||
343 | |||
344 | -o option | ||
345 | Can be used to give options in the format used in the configura- | ||
346 | tion file. This is useful for specifying options for which there | ||
347 | is no separate command-line flag. For full details of the op- | ||
348 | tions listed below, and their possible values, see ssh_config(5). | ||
349 | |||
350 | AddressFamily | ||
351 | BatchMode | ||
352 | BindAddress | ||
353 | ChallengeResponseAuthentication | ||
354 | CheckHostIP | ||
355 | Cipher | ||
356 | Ciphers | ||
357 | ClearAllForwardings | ||
358 | Compression | ||
359 | CompressionLevel | ||
360 | ConnectionAttempts | ||
361 | ConnectionTimeout | ||
362 | DynamicForward | ||
363 | EscapeChar | ||
364 | ForwardAgent | ||
365 | ForwardX11 | ||
366 | ForwardX11Trusted | ||
367 | GatewayPorts | ||
368 | GlobalKnownHostsFile | ||
369 | GSSAPIAuthentication | ||
370 | GSSAPIDelegateCredentials | ||
371 | Host | ||
372 | HostbasedAuthentication | ||
373 | HostKeyAlgorithms | ||
374 | HostKeyAlias | ||
375 | HostName | ||
376 | IdentityFile | ||
377 | LocalForward | ||
378 | LogLevel | ||
379 | MACs | ||
380 | NoHostAuthenticationForLocalhost | ||
381 | NumberOfPasswordPrompts | ||
382 | PasswordAuthentication | ||
383 | Port | ||
384 | PreferredAuthentications | ||
385 | Protocol | ||
386 | ProxyCommand | ||
387 | PubkeyAuthentication | ||
388 | RemoteForward | ||
389 | RhostsRSAAuthentication | ||
390 | RSAAuthentication | ||
391 | ServerAliveInterval | ||
392 | ServerAliveCountMax | ||
393 | SmartcardDevice | ||
394 | StrictHostKeyChecking | ||
395 | TCPKeepAlive | ||
396 | UsePrivilegedPort | ||
397 | User | ||
398 | UserKnownHostsFile | ||
399 | VerifyHostKeyDNS | ||
400 | XAuthLocation | ||
401 | |||
402 | -p port | ||
403 | Port to connect to on the remote host. This can be specified on | ||
404 | a per-host basis in the configuration file. | ||
405 | |||
406 | -q Quiet mode. Causes all warning and diagnostic messages to be | ||
407 | suppressed. | ||
408 | |||
409 | -R port:host:hostport | ||
410 | Specifies that the given port on the remote (server) host is to | ||
411 | be forwarded to the given host and port on the local side. This | ||
412 | works by allocating a socket to listen to port on the remote | ||
413 | side, and whenever a connection is made to this port, the connec- | ||
414 | tion is forwarded over the secure channel, and a connection is | ||
415 | made to host port hostport from the local machine. Port forward- | ||
416 | ings can also be specified in the configuration file. Privileged | ||
417 | ports can be forwarded only when logging in as root on the remote | ||
418 | machine. IPv6 addresses can be specified with an alternative | ||
419 | syntax: port/host/hostport. | ||
420 | |||
421 | -s May be used to request invocation of a subsystem on the remote | ||
422 | system. Subsystems are a feature of the SSH2 protocol which fa- | ||
423 | cilitate the use of SSH as a secure transport for other applica- | ||
424 | tions (eg. sftp(1)). The subsystem is specified as the remote | ||
425 | command. | ||
426 | |||
427 | -T Disable pseudo-tty allocation. | ||
428 | |||
429 | -t Force pseudo-tty allocation. This can be used to execute arbi- | ||
430 | trary screen-based programs on a remote machine, which can be | ||
431 | very useful, e.g., when implementing menu services. Multiple -t | ||
432 | options force tty allocation, even if ssh has no local tty. | ||
433 | |||
434 | -V Display the version number and exit. | ||
435 | |||
436 | -v Verbose mode. Causes ssh to print debugging messages about its | ||
437 | progress. This is helpful in debugging connection, authentica- | ||
438 | tion, and configuration problems. Multiple -v options increase | ||
439 | the verbosity. The maximum is 3. | ||
440 | |||
441 | -X Enables X11 forwarding. This can also be specified on a per-host | ||
442 | basis in a configuration file. | ||
443 | |||
444 | X11 forwarding should be enabled with caution. Users with the | ||
445 | ability to bypass file permissions on the remote host (for the | ||
446 | user's X authorization database) can access the local X11 display | ||
447 | through the forwarded connection. An attacker may then be able | ||
448 | to perform activities such as keystroke monitoring. | ||
449 | |||
450 | -x Disables X11 forwarding. | ||
451 | |||
452 | -Y Enables trusted X11 forwarding. | ||
453 | |||
454 | CONFIGURATION FILES | ||
455 | ssh may additionally obtain configuration data from a per-user configura- | ||
456 | tion file and a system-wide configuration file. The file format and con- | ||
457 | figuration options are described in ssh_config(5). | ||
458 | |||
459 | ENVIRONMENT | ||
460 | ssh will normally set the following environment variables: | ||
461 | |||
462 | DISPLAY The DISPLAY variable indicates the location of the X11 server. | ||
463 | It is automatically set by ssh to point to a value of the form | ||
464 | ``hostname:n'' where hostname indicates the host where the shell | ||
465 | runs, and n is an integer >= 1. ssh uses this special value to | ||
466 | forward X11 connections over the secure channel. The user | ||
467 | should normally not set DISPLAY explicitly, as that will render | ||
468 | the X11 connection insecure (and will require the user to manu- | ||
469 | ally copy any required authorization cookies). | ||
470 | |||
471 | HOME Set to the path of the user's home directory. | ||
472 | |||
473 | LOGNAME Synonym for USER; set for compatibility with systems that use | ||
474 | this variable. | ||
475 | |||
476 | MAIL Set to the path of the user's mailbox. | ||
477 | |||
478 | PATH Set to the default PATH, as specified when compiling ssh. | ||
479 | |||
480 | SSH_ASKPASS | ||
481 | If ssh needs a passphrase, it will read the passphrase from the | ||
482 | current terminal if it was run from a terminal. If ssh does not | ||
483 | have a terminal associated with it but DISPLAY and SSH_ASKPASS | ||
484 | are set, it will execute the program specified by SSH_ASKPASS | ||
485 | and open an X11 window to read the passphrase. This is particu- | ||
486 | larly useful when calling ssh from a .Xsession or related | ||
487 | script. (Note that on some machines it may be necessary to | ||
488 | redirect the input from /dev/null to make this work.) | ||
489 | |||
490 | SSH_AUTH_SOCK | ||
491 | Identifies the path of a unix-domain socket used to communicate | ||
492 | with the agent. | ||
493 | |||
494 | SSH_CONNECTION | ||
495 | Identifies the client and server ends of the connection. The | ||
496 | variable contains four space-separated values: client ip-ad- | ||
497 | dress, client port number, server ip-address and server port | ||
498 | number. | ||
499 | |||
500 | SSH_ORIGINAL_COMMAND | ||
501 | The variable contains the original command line if a forced com- | ||
502 | mand is executed. It can be used to extract the original argu- | ||
503 | ments. | ||
504 | |||
505 | SSH_TTY This is set to the name of the tty (path to the device) associ- | ||
506 | ated with the current shell or command. If the current session | ||
507 | has no tty, this variable is not set. | ||
508 | |||
509 | TZ The timezone variable is set to indicate the present timezone if | ||
510 | it was set when the daemon was started (i.e., the daemon passes | ||
511 | the value on to new connections). | ||
512 | |||
513 | USER Set to the name of the user logging in. | ||
514 | |||
515 | Additionally, ssh reads $HOME/.ssh/environment, and adds lines of the | ||
516 | format ``VARNAME=value'' to the environment if the file exists and if | ||
517 | users are allowed to change their environment. For more information, see | ||
518 | the PermitUserEnvironment option in sshd_config(5). | ||
519 | |||
520 | FILES | ||
521 | $HOME/.ssh/known_hosts | ||
522 | Records host keys for all hosts the user has logged into that are | ||
523 | not in /etc/ssh/ssh_known_hosts. See sshd(8). | ||
524 | |||
525 | $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa | ||
526 | Contains the authentication identity of the user. They are for | ||
527 | protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively. | ||
528 | These files contain sensitive data and should be readable by the | ||
529 | user but not accessible by others (read/write/execute). Note | ||
530 | that ssh ignores a private key file if it is accessible by oth- | ||
531 | ers. It is possible to specify a passphrase when generating the | ||
532 | key; the passphrase will be used to encrypt the sensitive part of | ||
533 | this file using 3DES. | ||
534 | |||
535 | $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub | ||
536 | Contains the public key for authentication (public part of the | ||
537 | identity file in human-readable form). The contents of the | ||
538 | $HOME/.ssh/identity.pub file should be added to the file | ||
539 | $HOME/.ssh/authorized_keys on all machines where the user wishes | ||
540 | to log in using protocol version 1 RSA authentication. The con- | ||
541 | tents of the $HOME/.ssh/id_dsa.pub and $HOME/.ssh/id_rsa.pub file | ||
542 | should be added to $HOME/.ssh/authorized_keys on all machines | ||
543 | where the user wishes to log in using protocol version 2 DSA/RSA | ||
544 | authentication. These files are not sensitive and can (but need | ||
545 | not) be readable by anyone. These files are never used automati- | ||
546 | cally and are not necessary; they are only provided for the con- | ||
547 | venience of the user. | ||
548 | |||
549 | $HOME/.ssh/config | ||
550 | This is the per-user configuration file. The file format and | ||
551 | configuration options are described in ssh_config(5). | ||
552 | |||
553 | $HOME/.ssh/authorized_keys | ||
554 | Lists the public keys (RSA/DSA) that can be used for logging in | ||
555 | as this user. The format of this file is described in the | ||
556 | sshd(8) manual page. In the simplest form the format is the same | ||
557 | as the .pub identity files. This file is not highly sensitive, | ||
558 | but the recommended permissions are read/write for the user, and | ||
559 | not accessible by others. | ||
560 | |||
561 | /etc/ssh/ssh_known_hosts | ||
562 | Systemwide list of known host keys. This file should be prepared | ||
563 | by the system administrator to contain the public host keys of | ||
564 | all machines in the organization. This file should be world- | ||
565 | readable. This file contains public keys, one per line, in the | ||
566 | following format (fields separated by spaces): system name, pub- | ||
567 | lic key and optional comment field. When different names are | ||
568 | used for the same machine, all such names should be listed, sepa- | ||
569 | rated by commas. The format is described in the sshd(8) manual | ||
570 | page. | ||
571 | |||
572 | The canonical system name (as returned by name servers) is used | ||
573 | by sshd(8) to verify the client host when logging in; other names | ||
574 | are needed because ssh does not convert the user-supplied name to | ||
575 | a canonical name before checking the key, because someone with | ||
576 | access to the name servers would then be able to fool host au- | ||
577 | thentication. | ||
578 | |||
579 | /etc/ssh/ssh_config | ||
580 | Systemwide configuration file. The file format and configuration | ||
581 | options are described in ssh_config(5). | ||
582 | |||
583 | /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key, | ||
584 | /etc/ssh/ssh_host_rsa_key | ||
585 | These three files contain the private parts of the host keys and | ||
586 | are used for RhostsRSAAuthentication and HostbasedAuthentication. | ||
587 | If the protocol version 1 RhostsRSAAuthentication method is used, | ||
588 | ssh must be setuid root, since the host key is readable only by | ||
589 | root. For protocol version 2, ssh uses ssh-keysign(8) to access | ||
590 | the host keys for HostbasedAuthentication. This eliminates the | ||
591 | requirement that ssh be setuid root when that authentication | ||
592 | method is used. By default ssh is not setuid root. | ||
593 | |||
594 | $HOME/.rhosts | ||
595 | This file is used in rhosts authentication to list the host/user | ||
596 | pairs that are permitted to log in. (Note that this file is also | ||
597 | used by rlogin and rsh, which makes using this file insecure.) | ||
598 | Each line of the file contains a host name (in the canonical form | ||
599 | returned by name servers), and then a user name on that host, | ||
600 | separated by a space. On some machines this file may need to be | ||
601 | world-readable if the user's home directory is on a NFS parti- | ||
602 | tion, because sshd(8) reads it as root. Additionally, this file | ||
603 | must be owned by the user, and must not have write permissions | ||
604 | for anyone else. The recommended permission for most machines is | ||
605 | read/write for the user, and not accessible by others. | ||
606 | |||
607 | Note that by default sshd(8) will be installed so that it re- | ||
608 | quires successful RSA host authentication before permitting | ||
609 | rhosts authentication. If the server machine does not have the | ||
610 | client's host key in /etc/ssh/ssh_known_hosts, it can be stored | ||
611 | in $HOME/.ssh/known_hosts. The easiest way to do this is to con- | ||
612 | nect back to the client from the server machine using ssh; this | ||
613 | will automatically add the host key to $HOME/.ssh/known_hosts. | ||
614 | |||
615 | $HOME/.shosts | ||
616 | This file is used exactly the same way as .rhosts. The purpose | ||
617 | for having this file is to be able to use rhosts authentication | ||
618 | with ssh without permitting login with rlogin or rsh(1). | ||
619 | |||
620 | /etc/hosts.equiv | ||
621 | This file is used during rhosts authentication. It contains | ||
622 | canonical hosts names, one per line (the full format is described | ||
623 | in the sshd(8) manual page). If the client host is found in this | ||
624 | file, login is automatically permitted provided client and server | ||
625 | user names are the same. Additionally, successful RSA host au- | ||
626 | thentication is normally required. This file should only be | ||
627 | writable by root. | ||
628 | |||
629 | /etc/shosts.equiv | ||
630 | This file is processed exactly as /etc/hosts.equiv. This file | ||
631 | may be useful to permit logins using ssh but not using | ||
632 | rsh/rlogin. | ||
633 | |||
634 | /etc/ssh/sshrc | ||
635 | Commands in this file are executed by ssh when the user logs in | ||
636 | just before the user's shell (or command) is started. See the | ||
637 | sshd(8) manual page for more information. | ||
638 | |||
639 | $HOME/.ssh/rc | ||
640 | Commands in this file are executed by ssh when the user logs in | ||
641 | just before the user's shell (or command) is started. See the | ||
642 | sshd(8) manual page for more information. | ||
643 | |||
644 | $HOME/.ssh/environment | ||
645 | Contains additional definitions for environment variables, see | ||
646 | section ENVIRONMENT above. | ||
647 | |||
648 | DIAGNOSTICS | ||
649 | ssh exits with the exit status of the remote command or with 255 if an | ||
650 | error occurred. | ||
651 | |||
652 | SEE ALSO | ||
653 | gzip(1), rsh(1), scp(1), sftp(1), ssh-add(1), ssh-agent(1), | ||
654 | ssh-keygen(1), telnet(1), hosts.equiv(5), ssh_config(5), ssh-keysign(8), | ||
655 | sshd(8) | ||
656 | |||
657 | T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH | ||
658 | Protocol Architecture, draft-ietf-secsh-architecture-12.txt, January | ||
659 | 2002, work in progress material. | ||
660 | |||
661 | AUTHORS | ||
662 | OpenSSH is a derivative of the original and free ssh 1.2.12 release by | ||
663 | Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo | ||
664 | de Raadt and Dug Song removed many bugs, re-added newer features and | ||
665 | created OpenSSH. Markus Friedl contributed the support for SSH protocol | ||
666 | versions 1.5 and 2.0. | ||
667 | |||
668 | OpenBSD 3.4 September 25, 1999 11 | ||