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1SSH(1) OpenBSD Reference Manual SSH(1)
2
3NAME
4 ssh - OpenSSH SSH client (remote login program)
5
6SYNOPSIS
7 ssh [-1246AaCfgkMNnqsTtVvXxY] [-b bind_address] [-c cipher_spec]
8 [-D port] [-e escape_char] [-F configfile] [-i identity_file]
9 [-L [bind_address:]port:host:hostport] [-l login_name] [-m mac_spec]
10 [-O ctl_cmd] [-o option] [-p port]
11 [-R [bind_address:]port:host:hostport] [-S ctl_path] [user@]hostname
12 [command]
13
14DESCRIPTION
15 ssh (SSH client) is a program for logging into a remote machine and for
16 executing commands on a remote machine. It is intended to replace rlogin
17 and rsh, and provide secure encrypted communications between two untrust-
18 ed hosts over an insecure network. X11 connections and arbitrary TCP/IP
19 ports can also be forwarded over the secure channel.
20
21 ssh connects and logs into the specified hostname (with optional user
22 name). The user must prove his/her identity to the remote machine using
23 one of several methods depending on the protocol version used.
24
25 If command is specified, command is executed on the remote host instead
26 of a login shell.
27
28 SSH protocol version 1
29 The first authentication method is the rhosts or hosts.equiv method com-
30 bined with RSA-based host authentication. If the machine the user logs
31 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote
32 machine, and the user names are the same on both sides, or if the files
33 $HOME/.rhosts or $HOME/.shosts exist in the user's home directory on the
34 remote machine and contain a line containing the name of the client ma-
35 chine and the name of the user on that machine, the user is considered
36 for log in. Additionally, if the server can verify the client's host key
37 (see /etc/ssh/ssh_known_hosts and $HOME/.ssh/known_hosts in the FILES
38 section), only then is login permitted. This authentication method clos-
39 es security holes due to IP spoofing, DNS spoofing and routing spoofing.
40 [Note to the administrator: /etc/hosts.equiv, $HOME/.rhosts, and the
41 rlogin/rsh protocol in general, are inherently insecure and should be
42 disabled if security is desired.]
43
44 As a second authentication method, ssh supports RSA based authentication.
45 The scheme is based on public-key cryptography: there are cryptosystems
46 where encryption and decryption are done using separate keys, and it is
47 not possible to derive the decryption key from the encryption key. RSA
48 is one such system. The idea is that each user creates a public/private
49 key pair for authentication purposes. The server knows the public key,
50 and only the user knows the private key.
51
52 The file $HOME/.ssh/authorized_keys lists the public keys that are per-
53 mitted for logging in. When the user logs in, the ssh program tells the
54 server which key pair it would like to use for authentication. The serv-
55 er checks if this key is permitted, and if so, sends the user (actually
56 the ssh program running on behalf of the user) a challenge, a random num-
57 ber, encrypted by the user's public key. The challenge can only be de-
58 crypted using the proper private key. The user's client then decrypts
59 the challenge using the private key, proving that he/she knows the pri-
60 vate key but without disclosing it to the server.
61
62 ssh implements the RSA authentication protocol automatically. The user
63 creates his/her RSA key pair by running ssh-keygen(1). This stores the
64 private key in $HOME/.ssh/identity and stores the public key in
65 $HOME/.ssh/identity.pub in the user's home directory. The user should
66 then copy the identity.pub to $HOME/.ssh/authorized_keys in his/her home
67 directory on the remote machine (the authorized_keys file corresponds to
68 the conventional $HOME/.rhosts file, and has one key per line, though the
69 lines can be very long). After this, the user can log in without giving
70 the password.
71
72 The most convenient way to use RSA authentication may be with an authen-
73 tication agent. See ssh-agent(1) for more information.
74
75 If other authentication methods fail, ssh prompts the user for a pass-
76 word. The password is sent to the remote host for checking; however,
77 since all communications are encrypted, the password cannot be seen by
78 someone listening on the network.
79
80 SSH protocol version 2
81 When a user connects using protocol version 2, similar authentication
82 methods are available. Using the default values for
83 PreferredAuthentications, the client will try to authenticate first using
84 the hostbased method; if this method fails, public key authentication is
85 attempted, and finally if this method fails, keyboard-interactive and
86 password authentication are tried.
87
88 The public key method is similar to RSA authentication described in the
89 previous section and allows the RSA or DSA algorithm to be used: The
90 client uses his private key, $HOME/.ssh/id_dsa or $HOME/.ssh/id_rsa, to
91 sign the session identifier and sends the result to the server. The
92 server checks whether the matching public key is listed in
93 $HOME/.ssh/authorized_keys and grants access if both the key is found and
94 the signature is correct. The session identifier is derived from a
95 shared Diffie-Hellman value and is only known to the client and the serv-
96 er.
97
98 If public key authentication fails or is not available, a password can be
99 sent encrypted to the remote host to prove the user's identity.
100
101 Additionally, ssh supports hostbased or challenge response authentica-
102 tion.
103
104 Protocol 2 provides additional mechanisms for confidentiality (the traf-
105 fic is encrypted using AES, 3DES, Blowfish, CAST128 or Arcfour) and in-
106 tegrity (hmac-md5, hmac-sha1, hmac-ripemd160). Note that protocol 1
107 lacks a strong mechanism for ensuring the integrity of the connection.
108
109 Login session and remote execution
110 When the user's identity has been accepted by the server, the server ei-
111 ther executes the given command, or logs into the machine and gives the
112 user a normal shell on the remote machine. All communication with the
113 remote command or shell will be automatically encrypted.
114
115 If a pseudo-terminal has been allocated (normal login session), the user
116 may use the escape characters noted below.
117
118 If no pseudo-tty has been allocated, the session is transparent and can
119 be used to reliably transfer binary data. On most systems, setting the
120 escape character to ``none'' will also make the session transparent even
121 if a tty is used.
122
123 The session terminates when the command or shell on the remote machine
124 exits and all X11 and TCP/IP connections have been closed. The exit sta-
125 tus of the remote program is returned as the exit status of ssh.
126
127 Escape Characters
128 When a pseudo-terminal has been requested, ssh supports a number of func-
129 tions through the use of an escape character.
130
131 A single tilde character can be sent as ~~ or by following the tilde by a
132 character other than those described below. The escape character must
133 always follow a newline to be interpreted as special. The escape charac-
134 ter can be changed in configuration files using the EscapeChar configura-
135 tion directive or on the command line by the -e option.
136
137 The supported escapes (assuming the default `~') are:
138
139 ~. Disconnect.
140
141 ~^Z Background ssh.
142
143 ~# List forwarded connections.
144
145 ~& Background ssh at logout when waiting for forwarded connection /
146 X11 sessions to terminate.
147
148 ~? Display a list of escape characters.
149
150 ~B Send a BREAK to the remote system (only useful for SSH protocol
151 version 2 and if the peer supports it).
152
153 ~C Open command line. Currently this allows the addition of port
154 forwardings using the -L and -R options (see below). It also al-
155 lows the cancellation of existing remote port-forwardings using
156 -KR hostport. Basic help is available, using the -h option.
157
158 ~R Request rekeying of the connection (only useful for SSH protocol
159 version 2 and if the peer supports it).
160
161 X11 and TCP forwarding
162 If the ForwardX11 variable is set to ``yes'' (or see the description of
163 the -X and -x options described later) and the user is using X11 (the
164 DISPLAY environment variable is set), the connection to the X11 display
165 is automatically forwarded to the remote side in such a way that any X11
166 programs started from the shell (or command) will go through the encrypt-
167 ed channel, and the connection to the real X server will be made from the
168 local machine. The user should not manually set DISPLAY. Forwarding of
169 X11 connections can be configured on the command line or in configuration
170 files.
171
172 The DISPLAY value set by ssh will point to the server machine, but with a
173 display number greater than zero. This is normal, and happens because
174 ssh creates a ``proxy'' X server on the server machine for forwarding the
175 connections over the encrypted channel.
176
177 ssh will also automatically set up Xauthority data on the server machine.
178 For this purpose, it will generate a random authorization cookie, store
179 it in Xauthority on the server, and verify that any forwarded connections
180 carry this cookie and replace it by the real cookie when the connection
181 is opened. The real authentication cookie is never sent to the server
182 machine (and no cookies are sent in the plain).
183
184 If the ForwardAgent variable is set to ``yes'' (or see the description of
185 the -A and -a options described later) and the user is using an authenti-
186 cation agent, the connection to the agent is automatically forwarded to
187 the remote side.
188
189 Forwarding of arbitrary TCP/IP connections over the secure channel can be
190 specified either on the command line or in a configuration file. One
191 possible application of TCP/IP forwarding is a secure connection to an
192 electronic purse; another is going through firewalls.
193
194 Server authentication
195 ssh automatically maintains and checks a database containing identifica-
196 tions for all hosts it has ever been used with. Host keys are stored in
197 $HOME/.ssh/known_hosts in the user's home directory. Additionally, the
198 file /etc/ssh/ssh_known_hosts is automatically checked for known hosts.
199 Any new hosts are automatically added to the user's file. If a host's
200 identification ever changes, ssh warns about this and disables password
201 authentication to prevent a trojan horse from getting the user's pass-
202 word. Another purpose of this mechanism is to prevent man-in-the-middle
203 attacks which could otherwise be used to circumvent the encryption. The
204 StrictHostKeyChecking option can be used to prevent logins to machines
205 whose host key is not known or has changed.
206
207 ssh can be configured to verify host identification using fingerprint re-
208 source records (SSHFP) published in DNS. The VerifyHostKeyDNS option can
209 be used to control how DNS lookups are performed. SSHFP resource records
210 can be generated using ssh-keygen(1).
211
212 The options are as follows:
213
214 -1 Forces ssh to try protocol version 1 only.
215
216 -2 Forces ssh to try protocol version 2 only.
217
218 -4 Forces ssh to use IPv4 addresses only.
219
220 -6 Forces ssh to use IPv6 addresses only.
221
222 -A Enables forwarding of the authentication agent connection. This
223 can also be specified on a per-host basis in a configuration
224 file.
225
226 Agent forwarding should be enabled with caution. Users with the
227 ability to bypass file permissions on the remote host (for the
228 agent's Unix-domain socket) can access the local agent through
229 the forwarded connection. An attacker cannot obtain key material
230 from the agent, however they can perform operations on the keys
231 that enable them to authenticate using the identities loaded into
232 the agent.
233
234 -a Disables forwarding of the authentication agent connection.
235
236 -b bind_address
237 Specify the interface to transmit from on machines with multiple
238 interfaces or aliased addresses.
239
240 -C Requests compression of all data (including stdin, stdout,
241 stderr, and data for forwarded X11 and TCP/IP connections). The
242 compression algorithm is the same used by gzip(1), and the
243 ``level'' can be controlled by the CompressionLevel option for
244 protocol version 1. Compression is desirable on modem lines and
245 other slow connections, but will only slow down things on fast
246 networks. The default value can be set on a host-by-host basis
247 in the configuration files; see the Compression option.
248
249 -c cipher_spec
250 Selects the cipher specification for encrypting the session.
251
252 Protocol version 1 allows specification of a single cipher. The
253 suported values are ``3des'', ``blowfish'' and ``des''. 3des
254 (triple-des) is an encrypt-decrypt-encrypt triple with three dif-
255 ferent keys. It is believed to be secure. blowfish is a fast
256 block cipher; it appears very secure and is much faster than
257 3des. des is only supported in the ssh client for interoperabil-
258 ity with legacy protocol 1 implementations that do not support
259 the 3des cipher. Its use is strongly discouraged due to crypto-
260 graphic weaknesses. The default is ``3des''.
261
262 For protocol version 2 cipher_spec is a comma-separated list of
263 ciphers listed in order of preference. The supported ciphers are
264 ``3des-cbc'', ``aes128-cbc'', ``aes192-cbc'', ``aes256-cbc'',
265 ``aes128-ctr'', ``aes192-ctr'', ``aes256-ctr'', ``arcfour'',
266 ``blowfish-cbc'', and ``cast128-cbc''. The default is
267
268 ``aes128-cbc,3des-cbc,blowfish-cbc,cast128-cbc,arcfour,
269 aes192-cbc,aes256-cbc''
270
271 -D port
272 Specifies a local ``dynamic'' application-level port forwarding.
273 This works by allocating a socket to listen to port on the local
274 side, and whenever a connection is made to this port, the connec-
275 tion is forwarded over the secure channel, and the application
276 protocol is then used to determine where to connect to from the
277 remote machine. Currently the SOCKS4 and SOCKS5 protocols are
278 supported, and ssh will act as a SOCKS server. Only root can
279 forward privileged ports. Dynamic port forwardings can also be
280 specified in the configuration file.
281
282 -e ch | ^ch | none
283 Sets the escape character for sessions with a pty (default: `~').
284 The escape character is only recognized at the beginning of a
285 line. The escape character followed by a dot (`.') closes the
286 connection; followed by control-Z suspends the connection; and
287 followed by itself sends the escape character once. Setting the
288 character to ``none'' disables any escapes and makes the session
289 fully transparent.
290
291 -F configfile
292 Specifies an alternative per-user configuration file. If a con-
293 figuration file is given on the command line, the system-wide
294 configuration file (/etc/ssh/ssh_config) will be ignored. The
295 default for the per-user configuration file is $HOME/.ssh/config.
296
297 -f Requests ssh to go to background just before command execution.
298 This is useful if ssh is going to ask for passwords or passphras-
299 es, but the user wants it in the background. This implies -n.
300 The recommended way to start X11 programs at a remote site is
301 with something like ssh -f host xterm.
302
303 -g Allows remote hosts to connect to local forwarded ports.
304
305 -I smartcard_device
306 Specifies which smartcard device to use. The argument is the de-
307 vice ssh should use to communicate with a smartcard used for
308 storing the user's private RSA key.
309
310 -i identity_file
311 Selects a file from which the identity (private key) for RSA or
312 DSA authentication is read. The default is $HOME/.ssh/identity
313 for protocol version 1, and $HOME/.ssh/id_rsa and
314 $HOME/.ssh/id_dsa for protocol version 2. Identity files may al-
315 so be specified on a per-host basis in the configuration file.
316 It is possible to have multiple -i options (and multiple identi-
317 ties specified in configuration files).
318
319 -k Disables forwarding (delegation) of GSSAPI credentials to the
320 server.
321
322 -L [bind_address:]port:host:hostport
323 Specifies that the given port on the local (client) host is to be
324 forwarded to the given host and port on the remote side. This
325 works by allocating a socket to listen to port on the local side,
326 optionally bound to the specified bind_address. Whenever a con-
327 nection is made to this port, the connection is forwarded over
328 the secure channel, and a connection is made to host port
329 hostport from the remote machine. Port forwardings can also be
330 specified in the configuration file. IPv6 addresses can be spec-
331 ified with an alternative syntax: [bind_address/]port/host/host-
332 port or by enclosing the address in square brackets. Only the
333 superuser can forward privileged ports. By default, the local
334 port is bound in accordance with the GatewayPorts setting. How-
335 ever, an explicit bind_address may be used to bind the connection
336 to a specific address. The bind_address of ``localhost'' indi-
337 cates that the listening port be bound for local use only, while
338 an empty address or `*' indicates that the port should be avail-
339 able from all interfaces.
340
341 -l login_name
342 Specifies the user to log in as on the remote machine. This also
343 may be specified on a per-host basis in the configuration file.
344
345 -M Places the ssh client into ``master'' mode for connection shar-
346 ing. Refer to the description of ControlMaster in ssh_config(5)
347 for details.
348
349 -m mac_spec
350 Additionally, for protocol version 2 a comma-separated list of
351 MAC (message authentication code) algorithms can be specified in
352 order of preference. See the MACs keyword for more information.
353
354 -N Do not execute a remote command. This is useful for just for-
355 warding ports (protocol version 2 only).
356
357 -n Redirects stdin from /dev/null (actually, prevents reading from
358 stdin). This must be used when ssh is run in the background. A
359 common trick is to use this to run X11 programs on a remote ma-
360 chine. For example, ssh -n shadows.cs.hut.fi emacs & will start
361 an emacs on shadows.cs.hut.fi, and the X11 connection will be au-
362 tomatically forwarded over an encrypted channel. The ssh program
363 will be put in the background. (This does not work if ssh needs
364 to ask for a password or passphrase; see also the -f option.)
365
366 -O ctl_cmd
367 Control an active connection multiplexing master process. When
368 the -O option is specified, the ctl_cmd argument is interpreted
369 and passed to the master process. Valid commands are: ``check''
370 (check that the master process is running) and ``exit'' (request
371 the master to exit).
372
373 -o option
374 Can be used to give options in the format used in the configura-
375 tion file. This is useful for specifying options for which there
376 is no separate command-line flag. For full details of the op-
377 tions listed below, and their possible values, see ssh_config(5).
378
379 AddressFamily
380 BatchMode
381 BindAddress
382 ChallengeResponseAuthentication
383 CheckHostIP
384 Cipher
385 Ciphers
386 ClearAllForwardings
387 Compression
388 CompressionLevel
389 ConnectionAttempts
390 ConnectTimeout
391 ControlMaster
392 ControlPath
393 DynamicForward
394 EscapeChar
395 ForwardAgent
396 ForwardX11
397 ForwardX11Trusted
398 GatewayPorts
399 GlobalKnownHostsFile
400 GSSAPIAuthentication
401 GSSAPIDelegateCredentials
402 HashKnownHosts
403 Host
404 HostbasedAuthentication
405 HostKeyAlgorithms
406 HostKeyAlias
407 HostName
408 IdentityFile
409 IdentitiesOnly
410 KbdInteractiveDevices
411 LocalForward
412 LogLevel
413 MACs
414 NoHostAuthenticationForLocalhost
415 NumberOfPasswordPrompts
416 PasswordAuthentication
417 Port
418 PreferredAuthentications
419 Protocol
420 ProxyCommand
421 PubkeyAuthentication
422 RemoteForward
423 RhostsRSAAuthentication
424 RSAAuthentication
425 SendEnv
426 ServerAliveInterval
427 ServerAliveCountMax
428 SmartcardDevice
429 StrictHostKeyChecking
430 TCPKeepAlive
431 UsePrivilegedPort
432 User
433 UserKnownHostsFile
434 VerifyHostKeyDNS
435 XAuthLocation
436
437 -p port
438 Port to connect to on the remote host. This can be specified on
439 a per-host basis in the configuration file.
440
441 -q Quiet mode. Causes all warning and diagnostic messages to be
442 suppressed.
443
444 -R [bind_address:]port:host:hostport
445 Specifies that the given port on the remote (server) host is to
446 be forwarded to the given host and port on the local side. This
447 works by allocating a socket to listen to port on the remote
448 side, and whenever a connection is made to this port, the connec-
449 tion is forwarded over the secure channel, and a connection is
450 made to host port hostport from the local machine.
451
452 Port forwardings can also be specified in the configuration file.
453 Privileged ports can be forwarded only when logging in as root on
454 the remote machine. IPv6 addresses can be specified by enclosing
455 the address in square braces or using an alternative syntax:
456 [bind_address/]host/port/hostport.
457
458 By default, the listening socket on the server will be bound to
459 the loopback interface only. This may be overriden by specifying
460 a bind_address. An empty bind_address, or the address `*', indi-
461 cates that the remote socket should listen on all interfaces.
462 Specifying a remote bind_address will only succeed if the serv-
463 er's GatewayPorts option is enabled (see sshd_config(5)).
464
465 -S ctl_path
466 Specifies the location of a control socket for connection shar-
467 ing. Refer to the description of ControlPath and ControlMaster
468 in ssh_config(5) for details.
469
470 -s May be used to request invocation of a subsystem on the remote
471 system. Subsystems are a feature of the SSH2 protocol which fa-
472 cilitate the use of SSH as a secure transport for other applica-
473 tions (eg. sftp(1)). The subsystem is specified as the remote
474 command.
475
476 -T Disable pseudo-tty allocation.
477
478 -t Force pseudo-tty allocation. This can be used to execute arbi-
479 trary screen-based programs on a remote machine, which can be
480 very useful, e.g., when implementing menu services. Multiple -t
481 options force tty allocation, even if ssh has no local tty.
482
483 -V Display the version number and exit.
484
485 -v Verbose mode. Causes ssh to print debugging messages about its
486 progress. This is helpful in debugging connection, authentica-
487 tion, and configuration problems. Multiple -v options increase
488 the verbosity. The maximum is 3.
489
490 -X Enables X11 forwarding. This can also be specified on a per-host
491 basis in a configuration file.
492
493 X11 forwarding should be enabled with caution. Users with the
494 ability to bypass file permissions on the remote host (for the
495 user's X authorization database) can access the local X11 display
496 through the forwarded connection. An attacker may then be able
497 to perform activities such as keystroke monitoring.
498
499 For this reason, X11 forwarding is subjected to X11 SECURITY ex-
500 tension restrictions by default. Please refer to the ssh -Y op-
501 tion and the ForwardX11Trusted directive in ssh_config(5) for
502 more information.
503
504 -x Disables X11 forwarding.
505
506 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
507 subjected to the X11 SECURITY extension controls.
508
509CONFIGURATION FILES
510 ssh may additionally obtain configuration data from a per-user configura-
511 tion file and a system-wide configuration file. The file format and con-
512 figuration options are described in ssh_config(5).
513
514ENVIRONMENT
515 ssh will normally set the following environment variables:
516
517 DISPLAY The DISPLAY variable indicates the location of the X11 server.
518 It is automatically set by ssh to point to a value of the form
519 ``hostname:n'' where hostname indicates the host where the shell
520 runs, and n is an integer >= 1. ssh uses this special value to
521 forward X11 connections over the secure channel. The user
522 should normally not set DISPLAY explicitly, as that will render
523 the X11 connection insecure (and will require the user to manu-
524 ally copy any required authorization cookies).
525
526 HOME Set to the path of the user's home directory.
527
528 LOGNAME Synonym for USER; set for compatibility with systems that use
529 this variable.
530
531 MAIL Set to the path of the user's mailbox.
532
533 PATH Set to the default PATH, as specified when compiling ssh.
534
535 SSH_ASKPASS
536 If ssh needs a passphrase, it will read the passphrase from the
537 current terminal if it was run from a terminal. If ssh does not
538 have a terminal associated with it but DISPLAY and SSH_ASKPASS
539 are set, it will execute the program specified by SSH_ASKPASS
540 and open an X11 window to read the passphrase. This is particu-
541 larly useful when calling ssh from a .xsession or related
542 script. (Note that on some machines it may be necessary to
543 redirect the input from /dev/null to make this work.)
544
545 SSH_AUTH_SOCK
546 Identifies the path of a unix-domain socket used to communicate
547 with the agent.
548
549 SSH_CONNECTION
550 Identifies the client and server ends of the connection. The
551 variable contains four space-separated values: client ip-ad-
552 dress, client port number, server ip-address and server port
553 number.
554
555 SSH_ORIGINAL_COMMAND
556 The variable contains the original command line if a forced com-
557 mand is executed. It can be used to extract the original argu-
558 ments.
559
560 SSH_TTY This is set to the name of the tty (path to the device) associ-
561 ated with the current shell or command. If the current session
562 has no tty, this variable is not set.
563
564 TZ The timezone variable is set to indicate the present timezone if
565 it was set when the daemon was started (i.e., the daemon passes
566 the value on to new connections).
567
568 USER Set to the name of the user logging in.
569
570 Additionally, ssh reads $HOME/.ssh/environment, and adds lines of the
571 format ``VARNAME=value'' to the environment if the file exists and if
572 users are allowed to change their environment. For more information, see
573 the PermitUserEnvironment option in sshd_config(5).
574
575FILES
576 $HOME/.ssh/known_hosts
577 Records host keys for all hosts the user has logged into that are
578 not in /etc/ssh/ssh_known_hosts. See sshd(8).
579
580 $HOME/.ssh/identity, $HOME/.ssh/id_dsa, $HOME/.ssh/id_rsa
581 Contains the authentication identity of the user. They are for
582 protocol 1 RSA, protocol 2 DSA, and protocol 2 RSA, respectively.
583 These files contain sensitive data and should be readable by the
584 user but not accessible by others (read/write/execute). Note
585 that ssh ignores a private key file if it is accessible by oth-
586 ers. It is possible to specify a passphrase when generating the
587 key; the passphrase will be used to encrypt the sensitive part of
588 this file using 3DES.
589
590 $HOME/.ssh/identity.pub, $HOME/.ssh/id_dsa.pub, $HOME/.ssh/id_rsa.pub
591 Contains the public key for authentication (public part of the
592 identity file in human-readable form). The contents of the
593 $HOME/.ssh/identity.pub file should be added to the file
594 $HOME/.ssh/authorized_keys on all machines where the user wishes
595 to log in using protocol version 1 RSA authentication. The con-
596 tents of the $HOME/.ssh/id_dsa.pub and $HOME/.ssh/id_rsa.pub file
597 should be added to $HOME/.ssh/authorized_keys on all machines
598 where the user wishes to log in using protocol version 2 DSA/RSA
599 authentication. These files are not sensitive and can (but need
600 not) be readable by anyone. These files are never used automati-
601 cally and are not necessary; they are only provided for the con-
602 venience of the user.
603
604 $HOME/.ssh/config
605 This is the per-user configuration file. The file format and
606 configuration options are described in ssh_config(5). Because of
607 the potential for abuse, this file must have strict permissions:
608 read/write for the user, and not accessible by others.
609
610 $HOME/.ssh/authorized_keys
611 Lists the public keys (RSA/DSA) that can be used for logging in
612 as this user. The format of this file is described in the
613 sshd(8) manual page. In the simplest form the format is the same
614 as the .pub identity files. This file is not highly sensitive,
615 but the recommended permissions are read/write for the user, and
616 not accessible by others.
617
618 /etc/ssh/ssh_known_hosts
619 Systemwide list of known host keys. This file should be prepared
620 by the system administrator to contain the public host keys of
621 all machines in the organization. This file should be world-
622 readable. This file contains public keys, one per line, in the
623 following format (fields separated by spaces): system name, pub-
624 lic key and optional comment field. When different names are
625 used for the same machine, all such names should be listed, sepa-
626 rated by commas. The format is described in the sshd(8) manual
627 page.
628
629 The canonical system name (as returned by name servers) is used
630 by sshd(8) to verify the client host when logging in; other names
631 are needed because ssh does not convert the user-supplied name to
632 a canonical name before checking the key, because someone with
633 access to the name servers would then be able to fool host au-
634 thentication.
635
636 /etc/ssh/ssh_config
637 Systemwide configuration file. The file format and configuration
638 options are described in ssh_config(5).
639
640 /etc/ssh/ssh_host_key, /etc/ssh/ssh_host_dsa_key,
641 /etc/ssh/ssh_host_rsa_key
642 These three files contain the private parts of the host keys and
643 are used for RhostsRSAAuthentication and HostbasedAuthentication.
644 If the protocol version 1 RhostsRSAAuthentication method is used,
645 ssh must be setuid root, since the host key is readable only by
646 root. For protocol version 2, ssh uses ssh-keysign(8) to access
647 the host keys for HostbasedAuthentication. This eliminates the
648 requirement that ssh be setuid root when that authentication
649 method is used. By default ssh is not setuid root.
650
651 $HOME/.rhosts
652 This file is used in RhostsRSAAuthentication and
653 HostbasedAuthentication authentication to list the host/user
654 pairs that are permitted to log in. (Note that this file is also
655 used by rlogin and rsh, which makes using this file insecure.)
656 Each line of the file contains a host name (in the canonical form
657 returned by name servers), and then a user name on that host,
658 separated by a space. On some machines this file may need to be
659 world-readable if the user's home directory is on a NFS parti-
660 tion, because sshd(8) reads it as root. Additionally, this file
661 must be owned by the user, and must not have write permissions
662 for anyone else. The recommended permission for most machines is
663 read/write for the user, and not accessible by others.
664
665 Note that sshd(8) allows authentication only in combination with
666 client host key authentication before permitting log in. If the
667 server machine does not have the client's host key in
668 /etc/ssh/ssh_known_hosts, it can be stored in
669 $HOME/.ssh/known_hosts. The easiest way to do this is to connect
670 back to the client from the server machine using ssh; this will
671 automatically add the host key to $HOME/.ssh/known_hosts.
672
673 $HOME/.shosts
674 This file is used exactly the same way as .rhosts. The purpose
675 for having this file is to be able to use RhostsRSAAuthentication
676 and HostbasedAuthentication authentication without permitting lo-
677 gin with rlogin or rsh(1).
678
679 /etc/hosts.equiv
680 This file is used during RhostsRSAAuthentication and
681 HostbasedAuthentication authentication. It contains canonical
682 hosts names, one per line (the full format is described in the
683 sshd(8) manual page). If the client host is found in this file,
684 login is automatically permitted provided client and server user
685 names are the same. Additionally, successful client host key au-
686 thentication is required. This file should only be writable by
687 root.
688
689 /etc/shosts.equiv
690 This file is processed exactly as /etc/hosts.equiv. This file
691 may be useful to permit logins using ssh but not using
692 rsh/rlogin.
693
694 /etc/ssh/sshrc
695 Commands in this file are executed by ssh when the user logs in
696 just before the user's shell (or command) is started. See the
697 sshd(8) manual page for more information.
698
699 $HOME/.ssh/rc
700 Commands in this file are executed by ssh when the user logs in
701 just before the user's shell (or command) is started. See the
702 sshd(8) manual page for more information.
703
704 $HOME/.ssh/environment
705 Contains additional definitions for environment variables, see
706 section ENVIRONMENT above.
707
708DIAGNOSTICS
709 ssh exits with the exit status of the remote command or with 255 if an
710 error occurred.
711
712SEE ALSO
713 gzip(1), rsh(1), scp(1), sftp(1), ssh-add(1), ssh-agent(1),
714 ssh-keygen(1), telnet(1), hosts.equiv(5), ssh_config(5), ssh-keysign(8),
715 sshd(8)
716
717 T. Ylonen, T. Kivinen, M. Saarinen, T. Rinne, and S. Lehtinen, SSH
718 Protocol Architecture, draft-ietf-secsh-architecture-12.txt, January
719 2002, work in progress material.
720
721AUTHORS
722 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
723 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
724 de Raadt and Dug Song removed many bugs, re-added newer features and
725 created OpenSSH. Markus Friedl contributed the support for SSH protocol
726 versions 1.5 and 2.0.
727
728OpenBSD 3.6 September 25, 1999 11
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