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1
2SSH(1) System Reference Manual SSH(1)
3
4NAME
5 ssh - OpenSSH secure shell client (remote login program)
6
7SYNOPSIS
8 ssh [-l login_name] [hostname | user@hostname] [command]
9
10 ssh [-afgknqtvxACNPTX246] [-c cipher_spec] [-e escape_char] [-i
11 identity_file] [-l login_name] [-o option] [-p port] [-L
12 port:host:hostport] [-R port:host:hostport] [hostname |
13 user@hostname] [command]
14
15DESCRIPTION
16 ssh (Secure Shell) is a program for logging into a remote machine and for
17 executing commands on a remote machine. It is intended to replace rlogin
18 and rsh, and provide secure encrypted communications between two untrust-
19 ed hosts over an insecure network. X11 connections and arbitrary TCP/IP
20 ports can also be forwarded over the secure channel.
21
22 ssh connects and logs into the specified hostname. The user must prove
23 his/her identity to the remote machine using one of several methods de-
24 pending on the protocol version used:
25
26 SSH protocol version 1
27
28 First, if the machine the user logs in from is listed in /etc/hosts.equiv
29 or /etc/shosts.equiv on the remote machine, and the user names are the
30 same on both sides, the user is immediately permitted to log in. Second,
31 if .rhosts or .shosts exists in the user's home directory on the remote
32 machine and contains a line containing the name of the client machine and
33 the name of the user on that machine, the user is permitted to log in.
34 This form of authentication alone is normally not allowed by the server
35 because it is not secure.
36
37 The second (and primary) authentication method is the rhosts or
38 hosts.equiv method combined with RSA-based host authentication. It means
39 that if the login would be permitted by $HOME/.rhosts, $HOME/.shosts,
40 /etc/hosts.equiv, or /etc/shosts.equiv, and if additionally the server
41 can verify the client's host key (see /etc/ssh_known_hosts and
42 $HOME/.ssh/known_hosts in the FILES section), only then login is permit-
43 ted. This authentication method closes security holes due to IP spoof-
44 ing, DNS spoofing and routing spoofing. [Note to the administrator:
45 /etc/hosts.equiv, $HOME/.rhosts, and the rlogin/rsh protocol in general,
46 are inherently insecure and should be disabled if security is desired.]
47
48 As a third authentication method, ssh supports RSA based authentication.
49 The scheme is based on public-key cryptography: there are cryptosystems
50 where encryption and decryption are done using separate keys, and it is
51 not possible to derive the decryption key from the encryption key. RSA
52 is one such system. The idea is that each user creates a public/private
53 key pair for authentication purposes. The server knows the public key,
54 and only the user knows the private key. The file
55 $HOME/.ssh/authorized_keys lists the public keys that are permitted for
56 logging in. When the user logs in, the ssh program tells the server
57 which key pair it would like to use for authentication. The server
58 checks if this key is permitted, and if so, sends the user (actually the
59 ssh program running on behalf of the user) a challenge, a random number,
60 encrypted by the user's public key. The challenge can only be decrypted
61 using the proper private key. The user's client then decrypts the chal-
62 lenge using the private key, proving that he/she knows the private key
63 but without disclosing it to the server.
64
65
66 ssh implements the RSA authentication protocol automatically. The user
67 creates his/her RSA key pair by running ssh-keygen(1). This stores the
68 private key in $HOME/.ssh/identity and the public key in
69 $HOME/.ssh/identity.pub in the user's home directory. The user should
70 then copy the identity.pub to $HOME/.ssh/authorized_keys in his/her home
71 directory on the remote machine (the authorized_keys file corresponds to
72 the conventional $HOME/.rhosts file, and has one key per line, though the
73 lines can be very long). After this, the user can log in without giving
74 the password. RSA authentication is much more secure than rhosts authen-
75 tication.
76
77 The most convenient way to use RSA authentication may be with an authen-
78 tication agent. See ssh-agent(1) for more information.
79
80 If other authentication methods fail, ssh prompts the user for a pass-
81 word. The password is sent to the remote host for checking; however,
82 since all communications are encrypted, the password cannot be seen by
83 someone listening on the network.
84
85 SSH protocol version 2
86
87 When a user connects using the protocol version 2 different authentica-
88 tion methods are available: At first, the client attempts to authenticate
89 using the public key method. If this method fails password authentica-
90 tion is tried.
91
92 The public key method is similar to RSA authentication described in the
93 previous section except that the DSA algorithm is used instead of the
94 patented RSA algorithm. The client uses his private DSA key
95 $HOME/.ssh/id_dsa to sign the session identifier and sends the result to
96 the server. The server checks whether the matching public key is listed
97 in $HOME/.ssh/authorized_keys2 and grants access if both the key is found
98 and 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 for proving the user's identity. This
104 protocol 2 implementation does not yet support Kerberos or S/Key authen-
105 tication.
106
107 Protocol 2 provides additional mechanisms for confidentiality (the traf-
108 fic is encrypted using 3DES, Blowfish, CAST128 or Arcfour) and integrity
109 (hmac-sha1, hmac-md5). Note that protocol 1 lacks a strong mechanism for
110 ensuring the integrity of the connection.
111
112 Login session and remote execution
113
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 can disconnect with ~., and suspend ssh with ~^Z. All forwarded connec-
121 tions can be listed with ~# and if the session blocks waiting for for-
122 warded X11 or TCP/IP connections to terminate, it can be backgrounded
123 with ~& (this should not be used while the user shell is active, as it
124 can cause the shell to hang). All available escapes can be listed with
125 ~?.
126
127 A single tilde character can be sent as ~~ (or by following the tilde by
128 a character other than those described above). The escape character must
129 always follow a newline to be interpreted as special. The escape charac-
130 ter can be changed in configuration files or on the command line.
131
132
133 If no pseudo tty has been allocated, the session is transparent and can
134 be used to reliably transfer binary data. On most systems, setting the
135 escape character to ``none'' will also make the session transparent even
136 if a tty is used.
137
138 The session terminates when the command or shell in on the remote machine
139 exists and all X11 and TCP/IP connections have been closed. The exit
140 status of the remote program is returned as the exit status of ssh.
141
142 X11 and TCP forwarding
143
144 If the user is using X11 (the DISPLAY environment variable is set), the
145 connection to the X11 display is automatically forwarded to the remote
146 side in such a way that any X11 programs started from the shell (or com-
147 mand) will go through the encrypted channel, and the connection to the
148 real X server will be made from the local machine. The user should not
149 manually set DISPLAY. Forwarding of X11 connections can be configured on
150 the command line or in configuration files.
151
152 The DISPLAY value set by ssh will point to the server machine, but with a
153 display number greater than zero. This is normal, and happens because
154 ssh creates a ``proxy'' X server on the server machine for forwarding the
155 connections over the encrypted channel.
156
157 ssh will also automatically set up Xauthority data on the server machine.
158 For this purpose, it will generate a random authorization cookie, store
159 it in Xauthority on the server, and verify that any forwarded connections
160 carry this cookie and replace it by the real cookie when the connection
161 is opened. The real authentication cookie is never sent to the server
162 machine (and no cookies are sent in the plain).
163
164 If the user is using an authentication agent, the connection to the agent
165 is automatically forwarded to the remote side unless disabled on command
166 line or in a configuration file.
167
168 Forwarding of arbitrary TCP/IP connections over the secure channel can be
169 specified either on command line or in a configuration file. One possi-
170 ble application of TCP/IP forwarding is a secure connection to an elec-
171 tronic purse; another is going trough firewalls.
172
173 Server authentication
174
175 ssh automatically maintains and checks a database containing identifica-
176 tions for all hosts it has ever been used with. RSA host keys are stored
177 in $HOME/.ssh/known_hosts and DSA host keys are stored in
178 $HOME/.ssh/known_hosts2 in the user's home directory. Additionally, the
179 files /etc/ssh_known_hosts and /etc/ssh_known_hosts2 are automatically
180 checked for known hosts. Any new hosts are automatically added to the
181 user's file. If a host's identification ever changes, ssh warns about
182 this and disables password authentication to prevent a trojan horse from
183 getting the user's password. Another purpose of this mechanism is to
184 prevent man-in-the-middle attacks which could otherwise be used to cir-
185 cumvent the encryption. The StrictHostKeyChecking option (see below) can
186 be used to prevent logins to machines whose host key is not known or has
187 changed.
188
189OPTIONS
190 -a Disables forwarding of the authentication agent connection.
191
192 -A Enables forwarding of the authentication agent connection. This
193 can also be specified on a per-host basis in a configuration
194 file.
195
196 -c blowfish|3des
197 Selects the cipher to use for encrypting the session. 3des is
198 used by default. It is believed to be secure. 3des (triple-des)
199 is an encrypt-decrypt-encrypt triple with three different keys.
200 It is presumably more secure than the des cipher which is no
201 longer supported in ssh. blowfish is a fast block cipher, it ap-
202 pears very secure and is much faster than 3des.
203
204 -c 3des-cbc,blowfish-cbc,arcfour,cast128-cbc
205 Additionally, for protocol version 2 a comma-separated list of
206 ciphers can be specified in order of preference. Protocol version
207 2 supports 3DES, Blowfish and CAST128 in CBC mode and Arcfour.
208
209 -e ch|^ch|none
210 Sets the escape character for sessions with a pty (default: `~').
211 The escape character is only recognized at the beginning of a
212 line. The escape character followed by a dot (`.') closes the
213 connection, followed by control-Z suspends the connection, and
214 followed by itself sends the escape character once. Setting the
215 character to ``none'' disables any escapes and makes the session
216 fully transparent.
217
218 -f Requests ssh to go to background just before command execution.
219 This is useful if ssh is going to ask for passwords or passphras-
220 es, but the user wants it in the background. This implies -n.
221 The recommended way to start X11 programs at a remote site is
222 with something like ssh -f host xterm.
223
224 -g Allows remote hosts to connect to local forwarded ports.
225
226 -i identity_file
227 Selects the file from which the identity (private key) for RSA
228 authentication is read. Default is $HOME/.ssh/identity in the
229 user's home directory. Identity files may also be specified on a
230 per-host basis in the configuration file. It is possible to have
231 multiple -i options (and multiple identities specified in config-
232 uration files).
233
234 -k Disables forwarding of Kerberos tickets and AFS tokens. This may
235 also be specified on a per-host basis in the configuration file.
236
237 -l login_name
238 Specifies the user to log in as on the remote machine. This also
239 may be specified on a per-host basis in the configuration file.
240
241 -n Redirects stdin from /dev/null (actually, prevents reading from
242 stdin). This must be used when ssh is run in the background. A
243 common trick is to use this to run X11 programs on a remote ma-
244 chine. For example, ssh -n shadows.cs.hut.fi emacs & will start
245 an emacs on shadows.cs.hut.fi, and the X11 connection will be au-
246 tomatically forwarded over an encrypted channel. The ssh program
247 will be put in the background. (This does not work if ssh needs
248 to ask for a password or passphrase; see also the -f option.)
249
250 -N Do not execute a remote command. This is usefull if you just
251 want to forward ports (protocol version 2 only).
252
253 -o option
254 Can be used to give options in the format used in the config
255 file. This is useful for specifying options for which there is
256 no separate command-line flag. The option has the same format as
257 a line in the configuration file.
258
259 -p port
260 Port to connect to on the remote host. This can be specified on
261 a per-host basis in the configuration file.
262
263 -P Use a non-privileged port for outgoing connections. This can be
264 used if your firewall does not permit connections from privileged
265 ports. Note that this option turns off RhostsAuthentication and
266 RhostsRSAAuthentication.
267
268 -q Quiet mode. Causes all warning and diagnostic messages to be
269 suppressed. Only fatal errors are displayed.
270
271 -t Force pseudo-tty allocation. This can be used to execute arbi-
272 trary screen-based programs on a remote machine, which can be
273 very useful, e.g., when implementing menu services.
274
275 -T Disable pseudo-tty allocation (protocol version 2 only).
276
277 -v Verbose mode. Causes ssh to print debugging messages about its
278 progress. This is helpful in debugging connection, authentica-
279 tion, and configuration problems. The verbose mode is also used
280 to display skey(1) challenges, if the user entered "s/key" as
281 password.
282
283 -x Disables X11 forwarding.
284
285 -X Enables X11 forwarding. This can also be specified on a per-host
286 basis in a configuration file.
287
288 -C Requests compression of all data (including stdin, stdout,
289 stderr, and data for forwarded X11 and TCP/IP connections). The
290 compression algorithm is the same used by gzip(1), and the
291 ``level'' can be controlled by the CompressionLevel option (see
292 below). Compression is desirable on modem lines and other slow
293 connections, but will only slow down things on fast networks.
294 The default value can be set on a host-by-host basis in the con-
295 figuration files; see the Compress option below.
296
297 -L port:host:hostport
298 Specifies that the given port on the local (client) host is to be
299 forwarded to the given host and port on the remote side. This
300 works by allocating a socket to listen to port on the local side,
301 and whenever a connection is made to this port, the connection is
302 forwarded over the secure channel, and a connection is made to
303 host port hostport from the remote machine. Port forwardings can
304 also be specified in the configuration file. Only root can for-
305 ward privileged ports. IPv6 addresses can be specified with an
306 alternative syntax: port/host/hostport
307
308 -R port:host:hostport
309 Specifies that the given port on the remote (server) host is to
310 be forwarded to the given host and port on the local side. This
311 works by allocating a socket to listen to port on the remote
312 side, and whenever a connection is made to this port, the connec-
313 tion is forwarded over the secure channel, and a connection is
314 made to host port hostport from the local machine. Port forward-
315 ings can also be specified in the configuration file. Privileged
316 ports can be forwarded only when logging in as root on the remote
317 machine.
318
319 -2 Forces ssh to try protocol version 2 only.
320
321 -4 Forces ssh to use IPv4 addresses only.
322
323 -6 Forces ssh to use IPv6 addresses only.
324
325CONFIGURATION FILES
326 ssh obtains configuration data from the following sources (in this or-
327 der): command line options, user's configuration file
328 ($HOME/.ssh/config), and system-wide configuration file
329 (/etc/ssh_config). For each parameter, the first obtained value will be
330 used. The configuration files contain sections bracketed by ``Host''
331 specifications, and that section is only applied for hosts that match one
332 of the patterns given in the specification. The matched host name is the
333 one given on the command line.
334
335 Since the first obtained value for each parameter is used, more host-spe-
336 cific declarations should be given near the beginning of the file, and
337 general defaults at the end.
338
339 The configuration file has the following format:
340
341 Empty lines and lines starting with `#' are comments.
342
343 Otherwise a line is of the format ``keyword arguments''. The possible
344 keywords and their meanings are as follows (note that the configuration
345 files are case-sensitive):
346
347 Host Restricts the following declarations (up to the next Host key-
348 word) to be only for those hosts that match one of the patterns
349 given after the keyword. `*' and `?' can be used as wildcards in
350 the patterns. A single `*' as a pattern can be used to provide
351 global defaults for all hosts. The host is the hostname argument
352 given on the command line (i.e., the name is not converted to a
353 canonicalized host name before matching).
354
355 AFSTokenPassing
356 Specifies whether to pass AFS tokens to remote host. The argu-
357 ment to this keyword must be ``yes'' or ``no''.
358
359 BatchMode
360 If set to ``yes'', passphrase/password querying will be disabled.
361 This option is useful in scripts and other batch jobs where you
362 have no user to supply the password. The argument must be
363 ``yes'' or ``no''.
364
365 CheckHostIP
366 If this flag is set to ``yes'', ssh will additionally check the
367 host ip address in the known_hosts file. This allows ssh to de-
368 tect if a host key changed due to DNS spoofing. If the option is
369 set to ``no'', the check will not be executed.
370
371 Cipher Specifies the cipher to use for encrypting the session. Current-
372 ly, ``blowfish'', and ``3des'' are supported. The default is
373 ``3des''.
374
375 Ciphers
376 Specifies the ciphers allowed for protocol version 2 in order of
377 preference. Multiple ciphers must be comma-separated. The de-
378 fault is ``3des-cbc,blowfish-cbc,arcfour,cast128-cbc''.
379
380 Compression
381 Specifies whether to use compression. The argument must be
382 ``yes'' or ``no''.
383
384 CompressionLevel
385 Specifies the compression level to use if compression is enable.
386 The argument must be an integer from 1 (fast) to 9 (slow, best).
387 The default level is 6, which is good for most applications. The
388 meaning of the values is the same as in gzip(1).
389
390 ConnectionAttempts
391 Specifies the number of tries (one per second) to make before
392 falling back to rsh or exiting. The argument must be an integer.
393 This may be useful in scripts if the connection sometimes fails.
394
395 DSAAuthentication
396 Specifies whether to try DSA authentication. The argument to
397 this keyword must be ``yes'' or ``no''. DSA authentication will
398 only be attempted if a DSA identity file exists. Note that this
399 option applies to protocol version 2 only.
400
401 EscapeChar
402 Sets the escape character (default: `~'). The escape character
403 can also be set on the command line. The argument should be a
404 single character, `^' followed by a letter, or ``none'' to dis-
405 able the escape character entirely (making the connection trans-
406 parent for binary data).
407
408 FallBackToRsh
409 Specifies that if connecting via ssh fails due to a connection
410 refused error (there is no sshd(8) listening on the remote host),
411 rsh(1) should automatically be used instead (after a suitable
412 warning about the session being unencrypted). The argument must
413 be ``yes'' or ``no''.
414
415 ForwardAgent
416 Specifies whether the connection to the authentication agent (if
417 any) will be forwarded to the remote machine. The argument must
418 be ``yes'' or ``no''. The default is ``no''.
419
420 ForwardX11
421 Specifies whether X11 connections will be automatically redirect-
422 ed over the secure channel and DISPLAY set. The argument must be
423 ``yes'' or ``no''. The default is ``no''.
424
425 GatewayPorts
426 Specifies whether remote hosts are allowed to connect to local
427 forwarded ports. The argument must be ``yes'' or ``no''. The de-
428 fault is ``no''.
429
430 GlobalKnownHostsFile
431 Specifies a file to use instead of /etc/ssh_known_hosts.
432
433 HostName
434 Specifies the real host name to log into. This can be used to
435 specify nicknames or abbreviations for hosts. Default is the
436 name given on the command line. Numeric IP addresses are also
437 permitted (both on the command line and in HostName specifica-
438 tions).
439
440 IdentityFile
441 Specifies the file from which the user's RSA authentication iden-
442 tity is read (default $HOME/.ssh/identity in the user's home di-
443 rectory). Additionally, any identities represented by the au-
444 thentication agent will be used for authentication. The file
445 name may use the tilde syntax to refer to a user's home directo-
446 ry. It is possible to have multiple identity files specified in
447 configuration files; all these identities will be tried in se-
448 quence.
449
450 IdentityFile2
451 Specifies the file from which the user's DSA authentication iden-
452 tity is read (default $HOME/.ssh/id_dsa in the user's home direc-
453 tory). The file name may use the tilde syntax to refer to a us-
454 er's home directory. It is possible to have multiple identity
455 files specified in configuration files; all these identities will
456 be tried in sequence.
457
458 KeepAlive
459 Specifies whether the system should send keepalive messages to
460 the other side. If they are sent, death of the connection or
461 crash of one of the machines will be properly noticed. However,
462 this means that connections will die if the route is down tem-
463 porarily, and some people find it annoying.
464
465 The default is ``yes'' (to send keepalives), and the client will
466 notice if the network goes down or the remote host dies. This is
467 important in scripts, and many users want it too.
468
469 To disable keepalives, the value should be set to ``no'' in both
470 the server and the client configuration files.
471
472 KerberosAuthentication
473 Specifies whether Kerberos authentication will be used. The ar-
474 gument to this keyword must be ``yes'' or ``no''.
475
476 KerberosTgtPassing
477 Specifies whether a Kerberos TGT will be forwarded to the server.
478 This will only work if the Kerberos server is actually an AFS
479 kaserver. The argument to this keyword must be ``yes'' or
480 ``no''.
481
482 LocalForward
483 Specifies that a TCP/IP port on the local machine be forwarded
484 over the secure channel to given host:port from the remote ma-
485 chine. The first argument must be a port number, and the second
486 must be host:port. Multiple forwardings may be specified, and
487 additional forwardings can be given on the command line. Only
488 the superuser can forward privileged ports.
489
490 LogLevel
491 Gives the verbosity level that is used when logging messages from
492 ssh. The possible values are: QUIET, FATAL, ERROR, INFO, VERBOSE
493 and DEBUG. The default is INFO.
494
495 NumberOfPasswordPrompts
496 Specifies the number of password prompts before giving up. The
497 argument to this keyword must be an integer. Default is 3.
498
499 PasswordAuthentication
500 Specifies whether to use password authentication. The argument
501 to this keyword must be ``yes'' or ``no''. Note that this option
502 applies to both protocol version 1 and 2.
503
504 Port Specifies the port number to connect on the remote host. Default
505 is 22.
506
507 Protocol
508 Specifies the protocol versions ssh should support in order of
509 preference. The possible values are ``1'' and ``2''. Multiple
510 versions must be comma-separated. The default is ``1,2''. This
511 means that ssh tries version 1 and falls back to version 2 if
512 version 1 is not available.
513
514 ProxyCommand
515 Specifies the command to use to connect to the server. The com-
516 mand string extends to the end of the line, and is executed with
517 /bin/sh. In the command string, `%h' will be substituted by the
518 host name to connect and `%p' by the port. The command can be
519 basically anything, and should read from its standard input and
520 write to its standard output. It should eventually connect an
521 sshd(8) server running on some machine, or execute sshd -i some-
522 where. Host key management will be done using the HostName of
523 the host being connected (defaulting to the name typed by the us-
524 er). Note that CheckHostIP is not available for connects with a
525 proxy command.
526
527
528
529
530 RemoteForward
531 Specifies that a TCP/IP port on the remote machine be forwarded
532 over the secure channel to given host:port from the local ma-
533 chine. The first argument must be a port number, and the second
534 must be host:port. Multiple forwardings may be specified, and
535 additional forwardings can be given on the command line. Only
536 the superuser can forward privileged ports.
537
538 RhostsAuthentication
539 Specifies whether to try rhosts based authentication. Note that
540 this declaration only affects the client side and has no effect
541 whatsoever on security. Disabling rhosts authentication may re-
542 duce authentication time on slow connections when rhosts authen-
543 tication is not used. Most servers do not permit RhostsAuthenti-
544 cation because it is not secure (see RhostsRSAAuthentication).
545 The argument to this keyword must be ``yes'' or ``no''.
546
547 RhostsRSAAuthentication
548 Specifies whether to try rhosts based authentication with RSA
549 host authentication. This is the primary authentication method
550 for most sites. The argument must be ``yes'' or ``no''.
551
552 RSAAuthentication
553 Specifies whether to try RSA authentication. The argument to
554 this keyword must be ``yes'' or ``no''. RSA authentication will
555 only be attempted if the identity file exists, or an authentica-
556 tion agent is running. Note that this option applies to protocol
557 version 1 only.
558
559 SkeyAuthentication
560 Specifies whether to use skey(1) authentication. The argument to
561 this keyword must be ``yes'' or ``no''. The default is ``no''.
562
563 StrictHostKeyChecking
564 If this flag is set to ``yes'', ssh ssh will never automatically
565 add host keys to the $HOME/.ssh/known_hosts and
566 $HOME/.ssh/known_hosts2 files, and refuses to connect hosts whose
567 host key has changed. This provides maximum protection against
568 trojan horse attacks. However, it can be somewhat annoying if
569 you don't have good /etc/ssh_known_hosts and
570 /etc/ssh_known_hosts2 files installed and frequently connect new
571 hosts. Basically this option forces the user to manually add any
572 new hosts. Normally this option is disabled, and new hosts will
573 automatically be added to the known host files. The host keys of
574 known hosts will be verified automatically in either case. The
575 argument must be ``yes'' or ``no''.
576
577 UsePrivilegedPort
578 Specifies whether to use a privileged port for outgoing connec-
579 tions. The argument must be ``yes'' or ``no''. The default is
580 ``yes''. Note that setting this option to ``no'' turns off
581 RhostsAuthentication and RhostsRSAAuthentication.
582
583 User Specifies the user to log in as. This can be useful if you have
584 a different user name on different machines. This saves the
585 trouble of having to remember to give the user name on the com-
586 mand line.
587
588 UserKnownHostsFile
589 Specifies a file to use instead of $HOME/.ssh/known_hosts.
590
591 UseRsh Specifies that rlogin/rsh should be used for this host. It is
592 possible that the host does not at all support the ssh protocol.
593 This causes ssh to immediately execute rsh(1). All other options
594 (except HostName) are ignored if this has been specified. The
595 argument must be ``yes'' or ``no''.
596
597 XAuthLocation
598 Specifies the location of the xauth(1) program. The default is
599 /usr/X11R6/bin/xauth.
600
601ENVIRONMENT
602 ssh will normally set the following environment variables:
603
604 DISPLAY
605 The DISPLAY variable indicates the location of the X11 server.
606 It is automatically set by ssh to point to a value of the form
607 ``hostname:n'' where hostname indicates the host where the shell
608 runs, and n is an integer >= 1. ssh uses this special value to
609 forward X11 connections over the secure channel. The user should
610 normally not set DISPLAY explicitly, as that will render the X11
611 connection insecure (and will require the user to manually copy
612 any required authorization cookies).
613
614 HOME Set to the path of the user's home directory.
615
616 LOGNAME
617 Synonym for USER; set for compatibility with systems that use
618 this variable.
619
620 MAIL Set to point the user's mailbox.
621
622 PATH Set to the default PATH, as specified when compiling ssh.
623
624 SSH_AUTH_SOCK
625 indicates the path of a unix-domain socket used to communicate
626 with the agent.
627
628 SSH_CLIENT
629 Identifies the client end of the connection. The variable con-
630 tains three space-separated values: client ip-address, client
631 port number, and server port number.
632
633 SSH_TTY
634 This is set to the name of the tty (path to the device) associat-
635 ed with the current shell or command. If the current session has
636 no tty, this variable is not set.
637
638 TZ The timezone variable is set to indicate the present timezone if
639 it was set when the daemon was started (e.i., the daemon passes
640 the value on to new connections).
641
642 USER Set to the name of the user logging in.
643
644 Additionally, ssh reads $HOME/.ssh/environment, and adds lines of the
645 format ``VARNAME=value'' to the environment.
646
647FILES
648 $HOME/.ssh/known_hosts
649 Records host keys for all hosts the user has logged into (that
650 are not in /etc/ssh_known_hosts). See sshd(8).
651
652 $HOME/.ssh/identity, $HOME/.ssh/id_dsa
653 Contains the RSA and the DSA authentication identity of the user.
654 These files contain sensitive data and should be readable by the
655 user but not accessible by others (read/write/execute). Note
656 that ssh ignores a private key file if it is accessible by oth-
657 ers. It is possible to specify a passphrase when generating the
658 key; the passphrase will be used to encrypt the sensitive part of
659
660 this file using 3DES.
661
662 $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub
663 Contains the public key for authentication (public part of the
664 identity file in human-readable form). The contents of the
665 $HOME/.ssh/identity.pub file should be added to
666 $HOME/.ssh/authorized_keys on all machines where you wish to log
667 in using RSA authentication. The contents of the
668 $HOME/.ssh/id_dsa.pub file should be added to
669 $HOME/.ssh/authorized_keys2 on all machines where you wish to log
670 in using DSA authentication. These files are not sensitive and
671 can (but need not) be readable by anyone. These files are never
672 used automatically and are not necessary; they is only provided
673 for the convenience of the user.
674
675 $HOME/.ssh/config
676 This is the per-user configuration file. The format of this file
677 is described above. This file is used by the ssh client. This
678 file does not usually contain any sensitive information, but the
679 recommended permissions are read/write for the user, and not ac-
680 cessible by others.
681
682 $HOME/.ssh/authorized_keys
683 Lists the RSA keys that can be used for logging in as this user.
684 The format of this file is described in the sshd(8) manual page.
685 In the simplest form the format is the same as the .pub identity
686 files (that is, each line contains the number of bits in modulus,
687 public exponent, modulus, and comment fields, separated by
688 spaces). This file is not highly sensitive, but the recommended
689 permissions are read/write for the user, and not accessible by
690 others.
691
692 $HOME/.ssh/authorized_keys2
693 Lists the DSA keys that can be used for logging in as this user.
694 This file is not highly sensitive, but the recommended permis-
695 sions are read/write for the user, and not accessible by others.
696
697 /etc/ssh_known_hosts, /etc/ssh_known_hosts2
698 Systemwide list of known host keys. /etc/ssh_known_hosts con-
699 tains RSA and /etc/ssh_known_hosts2 contains DSA keys. These
700 files should be prepared by the system administrator to contain
701 the public host keys of all machines in the organization. This
702 file should be world-readable. This file contains public keys,
703 one per line, in the following format (fields separated by
704 spaces): system name, number of bits in modulus, public exponent,
705 modulus, and optional comment field. When different names are
706 used for the same machine, all such names should be listed, sepa-
707 rated by commas. The format is described on the sshd(8) manual
708 page.
709
710 The canonical system name (as returned by name servers) is used
711 by sshd(8) to verify the client host when logging in; other names
712 are needed because ssh does not convert the user-supplied name to
713 a canonical name before checking the key, because someone with
714 access to the name servers would then be able to fool host au-
715 thentication.
716
717 /etc/ssh_config
718 Systemwide configuration file. This file provides defaults for
719 those values that are not specified in the user's configuration
720 file, and for those users who do not have a configuration file.
721 This file must be world-readable.
722
723 $HOME/.rhosts
724 This file is used in .rhosts authentication to list the host/user
725 pairs that are permitted to log in. (Note that this file is also
726 used by rlogin and rsh, which makes using this file insecure.)
727 Each line of the file contains a host name (in the canonical form
728 returned by name servers), and then a user name on that host,
729 separated by a space. One some machines this file may need to be
730 world-readable if the user's home directory is on a NFS parti-
731 tion, because sshd(8) reads it as root. Additionally, this file
732 must be owned by the user, and must not have write permissions
733 for anyone else. The recommended permission for most machines is
734 read/write for the user, and not accessible by others.
735
736 Note that by default sshd(8) will be installed so that it re-
737 quires successful RSA host authentication before permitting
738 .rhosts authentication. If your server machine does not have the
739 client's host key in /etc/ssh_known_hosts, you can store it in
740 $HOME/.ssh/known_hosts. The easiest way to do this is to connect
741 back to the client from the server machine using ssh; this will
742 automatically add the host key to $HOME/.ssh/known_hosts.
743
744 $HOME/.shosts
745 This file is used exactly the same way as .rhosts. The purpose
746 for having this file is to be able to use rhosts authentication
747 with ssh without permitting login with rlogin(1) or rsh(1).
748
749 /etc/hosts.equiv
750 This file is used during .rhosts authentication. It contains
751 canonical hosts names, one per line (the full format is described
752 on the sshd(8) manual page). If the client host is found in this
753 file, login is automatically permitted provided client and server
754 user names are the same. Additionally, successful RSA host au-
755 thentication is normally required. This file should only be
756 writable by root.
757
758 /etc/shosts.equiv
759 This file is processed exactly as /etc/hosts.equiv. This file may
760 be useful to permit logins using ssh but not using rsh/rlogin.
761
762 /etc/sshrc
763 Commands in this file are executed by ssh when the user logs in
764 just before the user's shell (or command) is started. See the
765 sshd(8) manual page for more information.
766
767 $HOME/.ssh/rc
768 Commands in this file are executed by ssh when the user logs in
769 just before the user's shell (or command) is started. See the
770 sshd(8) manual page for more information.
771
772 $HOME/.ssh/environment
773 Contains additional definitions for environment variables, see
774 section ENVIRONMENT above.
775
776 libcrypto.so.X.1
777 A version of this library which includes support for the RSA al-
778 gorithm is required for proper operation.
779
780AUTHOR
781 OpenSSH is a derivative of the original (free) ssh 1.2.12 release by Tatu
782 Ylonen, but with bugs removed and newer features re-added. Rapidly after
783 the 1.2.12 release, newer versions of the original ssh bore successively
784 more restrictive licenses, and thus demand for a free version was born.
785
786 This version of OpenSSH
787
788 o has all components of a restrictive nature (i.e., patents) directly
789 removed from the source code; any licensed or patented components are
790
791
792 chosen from external libraries.
793
794 o has been updated to support SSH protocol 1.5 and 2, making it compat-
795 ible with all other SSH clients and servers.
796
797 o contains added support for kerberos(8) authentication and ticket
798 passing.
799
800 o supports one-time password authentication with skey(1).
801
802 OpenSSH has been created by Aaron Campbell, Bob Beck, Markus Friedl,
803 Niels Provos, Theo de Raadt, and Dug Song.
804
805 The support for SSH protocol 2 was written by Markus Friedl.
806
807SEE ALSO
808 rlogin(1), rsh(1), scp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1),
809 telnet(1), sshd(8),
810
811BSD Experimental September 25, 1999 13