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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 [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec]
8 [-D [bind_address:]port] [-e escape_char] [-F configfile] [-I pkcs11]
9 [-i identity_file] [-L [bind_address:]port:host:hostport]
10 [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port]
11 [-R [bind_address:]port:host:hostport] [-S ctl_path] [-W host:port]
12 [-w local_tun[:remote_tun]] [user@]hostname [command]
13
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
18 untrusted hosts over an insecure network. X11 connections and arbitrary
19 TCP ports can also be forwarded over the secure channel.
20
21 ssh connects and logs into the specified hostname (with optional user
22 name). The user must prove his/her identity to the remote machine using
23 one of several methods depending on the protocol version used (see
24 below).
25
26 If command is specified, it is executed on the remote host instead of a
27 login shell.
28
29 The options are as follows:
30
31 -1 Forces ssh to try protocol version 1 only.
32
33 -2 Forces ssh to try protocol version 2 only.
34
35 -4 Forces ssh to use IPv4 addresses only.
36
37 -6 Forces ssh to use IPv6 addresses only.
38
39 -A Enables forwarding of the authentication agent connection. This
40 can also be specified on a per-host basis in a configuration
41 file.
42
43 Agent forwarding should be enabled with caution. Users with the
44 ability to bypass file permissions on the remote host (for the
45 agent's UNIX-domain socket) can access the local agent through
46 the forwarded connection. An attacker cannot obtain key material
47 from the agent, however they can perform operations on the keys
48 that enable them to authenticate using the identities loaded into
49 the agent.
50
51 -a Disables forwarding of the authentication agent connection.
52
53 -b bind_address
54 Use bind_address on the local machine as the source address of
55 the connection. Only useful on systems with more than one
56 address.
57
58 -C Requests compression of all data (including stdin, stdout,
59 stderr, and data for forwarded X11 and TCP connections). The
60 compression algorithm is the same used by gzip(1), and the
61 ``level'' can be controlled by the CompressionLevel option for
62 protocol version 1. Compression is desirable on modem lines and
63 other slow connections, but will only slow down things on fast
64 networks. The default value can be set on a host-by-host basis
65 in the configuration files; see the Compression option.
66
67 -c cipher_spec
68 Selects the cipher specification for encrypting the session.
69
70 Protocol version 1 allows specification of a single cipher. The
71 supported values are ``3des'', ``blowfish'', and ``des''. 3des
72 (triple-des) is an encrypt-decrypt-encrypt triple with three
73 different keys. It is believed to be secure. blowfish is a fast
74 block cipher; it appears very secure and is much faster than
75 3des. des is only supported in the ssh client for
76 interoperability with legacy protocol 1 implementations that do
77 not support the 3des cipher. Its use is strongly discouraged due
78 to cryptographic weaknesses. The default is ``3des''.
79
80 For protocol version 2, cipher_spec is a comma-separated list of
81 ciphers listed in order of preference. See the Ciphers keyword
82 in ssh_config(5) for more information.
83
84 -D [bind_address:]port
85 Specifies a local ``dynamic'' application-level port forwarding.
86 This works by allocating a socket to listen to port on the local
87 side, optionally bound to the specified bind_address. Whenever a
88 connection is made to this port, the connection is forwarded over
89 the secure channel, and the application protocol is then used to
90 determine where to connect to from the remote machine. Currently
91 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
92 as a SOCKS server. Only root can forward privileged ports.
93 Dynamic port forwardings can also be specified in the
94 configuration file.
95
96 IPv6 addresses can be specified by enclosing the address in
97 square brackets. Only the superuser can forward privileged
98 ports. By default, the local port is bound in accordance with
99 the GatewayPorts setting. However, an explicit bind_address may
100 be used to bind the connection to a specific address. The
101 bind_address of ``localhost'' indicates that the listening port
102 be bound for local use only, while an empty address or `*'
103 indicates that the port should be available from all interfaces.
104
105 -e escape_char
106 Sets the escape character for sessions with a pty (default: `~').
107 The escape character is only recognized at the beginning of a
108 line. The escape character followed by a dot (`.') closes the
109 connection; followed by control-Z suspends the connection; and
110 followed by itself sends the escape character once. Setting the
111 character to ``none'' disables any escapes and makes the session
112 fully transparent.
113
114 -F configfile
115 Specifies an alternative per-user configuration file. If a
116 configuration file is given on the command line, the system-wide
117 configuration file (/etc/ssh/ssh_config) will be ignored. The
118 default for the per-user configuration file is ~/.ssh/config.
119
120 -f Requests ssh to go to background just before command execution.
121 This is useful if ssh is going to ask for passwords or
122 passphrases, but the user wants it in the background. This
123 implies -n. The recommended way to start X11 programs at a
124 remote site is with something like ssh -f host xterm.
125
126 If the ExitOnForwardFailure configuration option is set to
127 ``yes'', then a client started with -f will wait for all remote
128 port forwards to be successfully established before placing
129 itself in the background.
130
131 -g Allows remote hosts to connect to local forwarded ports.
132
133 -I pkcs11
134 Specify the PKCS#11 shared library ssh should use to communicate
135 with a PKCS#11 token providing the user's private RSA key.
136
137 -i identity_file
138 Selects a file from which the identity (private key) for RSA or
139 DSA authentication is read. The default is ~/.ssh/identity for
140 protocol version 1, and ~/.ssh/id_rsa and ~/.ssh/id_dsa for
141 protocol version 2. Identity files may also be specified on a
142 per-host basis in the configuration file. It is possible to have
143 multiple -i options (and multiple identities specified in
144 configuration files). ssh will also try to load certificate
145 information from the filename obtained by appending -cert.pub to
146 identity filenames.
147
148 -K Enables GSSAPI-based authentication and forwarding (delegation)
149 of GSSAPI credentials to the server.
150
151 -k Disables forwarding (delegation) of GSSAPI credentials to the
152 server.
153
154 -L [bind_address:]port:host:hostport
155 Specifies that the given port on the local (client) host is to be
156 forwarded to the given host and port on the remote side. This
157 works by allocating a socket to listen to port on the local side,
158 optionally bound to the specified bind_address. Whenever a
159 connection is made to this port, the connection is forwarded over
160 the secure channel, and a connection is made to host port
161 hostport from the remote machine. Port forwardings can also be
162 specified in the configuration file. IPv6 addresses can be
163 specified by enclosing the address in square brackets. Only the
164 superuser can forward privileged ports. By default, the local
165 port is bound in accordance with the GatewayPorts setting.
166 However, an explicit bind_address may be used to bind the
167 connection to a specific address. The bind_address of
168 ``localhost'' indicates that the listening port be bound for
169 local use only, while an empty address or `*' indicates that the
170 port should be available from all interfaces.
171
172 -l login_name
173 Specifies the user to log in as on the remote machine. This also
174 may be specified on a per-host basis in the configuration file.
175
176 -M Places the ssh client into ``master'' mode for connection
177 sharing. Multiple -M options places ssh into ``master'' mode
178 with confirmation required before slave connections are accepted.
179 Refer to the description of ControlMaster in ssh_config(5) for
180 details.
181
182 -m mac_spec
183 Additionally, for protocol version 2 a comma-separated list of
184 MAC (message authentication code) algorithms can be specified in
185 order of preference. See the MACs keyword for more information.
186
187 -N Do not execute a remote command. This is useful for just
188 forwarding ports (protocol version 2 only).
189
190 -n Redirects stdin from /dev/null (actually, prevents reading from
191 stdin). This must be used when ssh is run in the background. A
192 common trick is to use this to run X11 programs on a remote
193 machine. For example, ssh -n shadows.cs.hut.fi emacs & will
194 start an emacs on shadows.cs.hut.fi, and the X11 connection will
195 be automatically forwarded over an encrypted channel. The ssh
196 program will be put in the background. (This does not work if
197 ssh needs to ask for a password or passphrase; see also the -f
198 option.)
199
200 -O ctl_cmd
201 Control an active connection multiplexing master process. When
202 the -O option is specified, the ctl_cmd argument is interpreted
203 and passed to the master process. Valid commands are: ``check''
204 (check that the master process is running), ``forward'' (request
205 forwardings without command execution) and ``exit'' (request the
206 master to exit).
207
208 -o option
209 Can be used to give options in the format used in the
210 configuration file. This is useful for specifying options for
211 which there is no separate command-line flag. For full details
212 of the options listed below, and their possible values, see
213 ssh_config(5).
214
215 AddressFamily
216 BatchMode
217 BindAddress
218 ChallengeResponseAuthentication
219 CheckHostIP
220 Cipher
221 Ciphers
222 ClearAllForwardings
223 Compression
224 CompressionLevel
225 ConnectionAttempts
226 ConnectTimeout
227 ControlMaster
228 ControlPath
229 DynamicForward
230 EscapeChar
231 ExitOnForwardFailure
232 ForwardAgent
233 ForwardX11
234 ForwardX11Trusted
235 GatewayPorts
236 GlobalKnownHostsFile
237 GSSAPIAuthentication
238 GSSAPIDelegateCredentials
239 HashKnownHosts
240 Host
241 HostbasedAuthentication
242 HostKeyAlgorithms
243 HostKeyAlias
244 HostName
245 IdentityFile
246 IdentitiesOnly
247 KbdInteractiveDevices
248 LocalCommand
249 LocalForward
250 LogLevel
251 MACs
252 NoHostAuthenticationForLocalhost
253 NumberOfPasswordPrompts
254 PasswordAuthentication
255 PermitLocalCommand
256 PKCS11Provider
257 Port
258 PreferredAuthentications
259 Protocol
260 ProxyCommand
261 PubkeyAuthentication
262 RekeyLimit
263 RemoteForward
264 RhostsRSAAuthentication
265 RSAAuthentication
266 SendEnv
267 ServerAliveInterval
268 ServerAliveCountMax
269 StrictHostKeyChecking
270 TCPKeepAlive
271 Tunnel
272 TunnelDevice
273 UsePrivilegedPort
274 User
275 UserKnownHostsFile
276 VerifyHostKeyDNS
277 VisualHostKey
278 XAuthLocation
279
280 -p port
281 Port to connect to on the remote host. This can be specified on
282 a per-host basis in the configuration file.
283
284 -q Quiet mode. Causes most warning and diagnostic messages to be
285 suppressed.
286
287 -R [bind_address:]port:host:hostport
288 Specifies that the given port on the remote (server) host is to
289 be forwarded to the given host and port on the local side. This
290 works by allocating a socket to listen to port on the remote
291 side, and whenever a connection is made to this port, the
292 connection is forwarded over the secure channel, and a connection
293 is made to host port hostport from the local machine.
294
295 Port forwardings can also be specified in the configuration file.
296 Privileged ports can be forwarded only when logging in as root on
297 the remote machine. IPv6 addresses can be specified by enclosing
298 the address in square braces.
299
300 By default, the listening socket on the server will be bound to
301 the loopback interface only. This may be overridden by
302 specifying a bind_address. An empty bind_address, or the address
303 `*', indicates that the remote socket should listen on all
304 interfaces. Specifying a remote bind_address will only succeed
305 if the server's GatewayPorts option is enabled (see
306 sshd_config(5)).
307
308 If the port argument is `0', the listen port will be dynamically
309 allocated on the server and reported to the client at run time.
310 When used together with -O forward the allocated port will be
311 printed to the standard output.
312
313 -S ctl_path
314 Specifies the location of a control socket for connection
315 sharing, or the string ``none'' to disable connection sharing.
316 Refer to the description of ControlPath and ControlMaster in
317 ssh_config(5) for details.
318
319 -s May be used to request invocation of a subsystem on the remote
320 system. Subsystems are a feature of the SSH2 protocol which
321 facilitate the use of SSH as a secure transport for other
322 applications (eg. sftp(1)). The subsystem is specified as the
323 remote command.
324
325 -T Disable pseudo-tty allocation.
326
327 -t Force pseudo-tty allocation. This can be used to execute
328 arbitrary screen-based programs on a remote machine, which can be
329 very useful, e.g. when implementing menu services. Multiple -t
330 options force tty allocation, even if ssh has no local tty.
331
332 -V Display the version number and exit.
333
334 -v Verbose mode. Causes ssh to print debugging messages about its
335 progress. This is helpful in debugging connection,
336 authentication, and configuration problems. Multiple -v options
337 increase the verbosity. The maximum is 3.
338
339 -W host:port
340 Requests that standard input and output on the client be
341 forwarded to host on port over the secure channel. Implies -N,
342 -T, ExitOnForwardFailure and ClearAllForwardings and works with
343 Protocol version 2 only.
344
345 -w local_tun[:remote_tun]
346 Requests tunnel device forwarding with the specified tun(4)
347 devices between the client (local_tun) and the server
348 (remote_tun).
349
350 The devices may be specified by numerical ID or the keyword
351 ``any'', which uses the next available tunnel device. If
352 remote_tun is not specified, it defaults to ``any''. See also
353 the Tunnel and TunnelDevice directives in ssh_config(5). If the
354 Tunnel directive is unset, it is set to the default tunnel mode,
355 which is ``point-to-point''.
356
357 -X Enables X11 forwarding. This can also be specified on a per-host
358 basis in a configuration file.
359
360 X11 forwarding should be enabled with caution. Users with the
361 ability to bypass file permissions on the remote host (for the
362 user's X authorization database) can access the local X11 display
363 through the forwarded connection. An attacker may then be able
364 to perform activities such as keystroke monitoring.
365
366 For this reason, X11 forwarding is subjected to X11 SECURITY
367 extension restrictions by default. Please refer to the ssh -Y
368 option and the ForwardX11Trusted directive in ssh_config(5) for
369 more information.
370
371 -x Disables X11 forwarding.
372
373 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
374 subjected to the X11 SECURITY extension controls.
375
376 -y Send log information using the syslog(3) system module. By
377 default this information is sent to stderr.
378
379 ssh may additionally obtain configuration data from a per-user
380 configuration file and a system-wide configuration file. The file format
381 and configuration options are described in ssh_config(5).
382
383 ssh exits with the exit status of the remote command or with 255 if an
384 error occurred.
385
386AUTHENTICATION
387 The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to
388 use protocol 2 only, though this can be changed via the Protocol option
389 in ssh_config(5) or the -1 and -2 options (see above). Both protocols
390 support similar authentication methods, but protocol 2 is the default
391 since it provides additional mechanisms for confidentiality (the traffic
392 is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and
393 integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160). Protocol 1
394 lacks a strong mechanism for ensuring the integrity of the connection.
395
396 The methods available for authentication are: GSSAPI-based
397 authentication, host-based authentication, public key authentication,
398 challenge-response authentication, and password authentication.
399 Authentication methods are tried in the order specified above, though
400 protocol 2 has a configuration option to change the default order:
401 PreferredAuthentications.
402
403 Host-based authentication works as follows: If the machine the user logs
404 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote
405 machine, and the user names are the same on both sides, or if the files
406 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote
407 machine and contain a line containing the name of the client machine and
408 the name of the user on that machine, the user is considered for login.
409 Additionally, the server must be able to verify the client's host key
410 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts,
411 below) for login to be permitted. This authentication method closes
412 security holes due to IP spoofing, DNS spoofing, and routing spoofing.
413 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the
414 rlogin/rsh protocol in general, are inherently insecure and should be
415 disabled if security is desired.]
416
417 Public key authentication works as follows: The scheme is based on
418 public-key cryptography, using cryptosystems where encryption and
419 decryption are done using separate keys, and it is unfeasible to derive
420 the decryption key from the encryption key. The idea is that each user
421 creates a public/private key pair for authentication purposes. The
422 server knows the public key, and only the user knows the private key.
423 ssh implements public key authentication protocol automatically, using
424 either the RSA or DSA algorithms. Protocol 1 is restricted to using only
425 RSA keys, but protocol 2 may use either. The HISTORY section of ssl(8)
426 contains a brief discussion of the two algorithms.
427
428 The file ~/.ssh/authorized_keys lists the public keys that are permitted
429 for logging in. When the user logs in, the ssh program tells the server
430 which key pair it would like to use for authentication. The client
431 proves that it has access to the private key and the server checks that
432 the corresponding public key is authorized to accept the account.
433
434 The user creates his/her key pair by running ssh-keygen(1). This stores
435 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol
436 2 DSA), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in
437 ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA), or
438 ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home directory. The
439 user should then copy the public key to ~/.ssh/authorized_keys in his/her
440 home directory on the remote machine. The authorized_keys file
441 corresponds to the conventional ~/.rhosts file, and has one key per line,
442 though the lines can be very long. After this, the user can log in
443 without giving the password.
444
445 A variation on public key authentication is available in the form of
446 certificate authentication: instead of a set of public/private keys,
447 signed certificates are used. This has the advantage that a single
448 trusted certification authority can be used in place of many
449 public/private keys. See the CERTIFICATES section of ssh-keygen(1) for
450 more information.
451
452 The most convenient way to use public key or certificate authentication
453 may be with an authentication agent. See ssh-agent(1) for more
454 information.
455
456 Challenge-response authentication works as follows: The server sends an
457 arbitrary "challenge" text, and prompts for a response. Protocol 2
458 allows multiple challenges and responses; protocol 1 is restricted to
459 just one challenge/response. Examples of challenge-response
460 authentication include BSD Authentication (see login.conf(5)) and PAM
461 (some non-OpenBSD systems).
462
463 Finally, if other authentication methods fail, ssh prompts the user for a
464 password. The password is sent to the remote host for checking; however,
465 since all communications are encrypted, the password cannot be seen by
466 someone listening on the network.
467
468 ssh automatically maintains and checks a database containing
469 identification for all hosts it has ever been used with. Host keys are
470 stored in ~/.ssh/known_hosts in the user's home directory. Additionally,
471 the file /etc/ssh/ssh_known_hosts is automatically checked for known
472 hosts. Any new hosts are automatically added to the user's file. If a
473 host's identification ever changes, ssh warns about this and disables
474 password authentication to prevent server spoofing or man-in-the-middle
475 attacks, which could otherwise be used to circumvent the encryption. The
476 StrictHostKeyChecking option can be used to control logins to machines
477 whose host key is not known or has changed.
478
479 When the user's identity has been accepted by the server, the server
480 either executes the given command, or logs into the machine and gives the
481 user a normal shell on the remote machine. All communication with the
482 remote command or shell will be automatically encrypted.
483
484 If a pseudo-terminal has been allocated (normal login session), the user
485 may use the escape characters noted below.
486
487 If no pseudo-tty has been allocated, the session is transparent and can
488 be used to reliably transfer binary data. On most systems, setting the
489 escape character to ``none'' will also make the session transparent even
490 if a tty is used.
491
492 The session terminates when the command or shell on the remote machine
493 exits and all X11 and TCP connections have been closed.
494
495ESCAPE CHARACTERS
496 When a pseudo-terminal has been requested, ssh supports a number of
497 functions through the use of an escape character.
498
499 A single tilde character can be sent as ~~ or by following the tilde by a
500 character other than those described below. The escape character must
501 always follow a newline to be interpreted as special. The escape
502 character can be changed in configuration files using the EscapeChar
503 configuration directive or on the command line by the -e option.
504
505 The supported escapes (assuming the default `~') are:
506
507 ~. Disconnect.
508
509 ~^Z Background ssh.
510
511 ~# List forwarded connections.
512
513 ~& Background ssh at logout when waiting for forwarded connection /
514 X11 sessions to terminate.
515
516 ~? Display a list of escape characters.
517
518 ~B Send a BREAK to the remote system (only useful for SSH protocol
519 version 2 and if the peer supports it).
520
521 ~C Open command line. Currently this allows the addition of port
522 forwardings using the -L, -R and -D options (see above). It also
523 allows the cancellation of existing remote port-forwardings using
524 -KR[bind_address:]port. !command allows the user to execute a
525 local command if the PermitLocalCommand option is enabled in
526 ssh_config(5). Basic help is available, using the -h option.
527
528 ~R Request rekeying of the connection (only useful for SSH protocol
529 version 2 and if the peer supports it).
530
531TCP FORWARDING
532 Forwarding of arbitrary TCP connections over the secure channel can be
533 specified either on the command line or in a configuration file. One
534 possible application of TCP forwarding is a secure connection to a mail
535 server; another is going through firewalls.
536
537 In the example below, we look at encrypting communication between an IRC
538 client and server, even though the IRC server does not directly support
539 encrypted communications. This works as follows: the user connects to
540 the remote host using ssh, specifying a port to be used to forward
541 connections to the remote server. After that it is possible to start the
542 service which is to be encrypted on the client machine, connecting to the
543 same local port, and ssh will encrypt and forward the connection.
544
545 The following example tunnels an IRC session from client machine
546 ``127.0.0.1'' (localhost) to remote server ``server.example.com'':
547
548 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
549 $ irc -c '#users' -p 1234 pinky 127.0.0.1
550
551 This tunnels a connection to IRC server ``server.example.com'', joining
552 channel ``#users'', nickname ``pinky'', using port 1234. It doesn't
553 matter which port is used, as long as it's greater than 1023 (remember,
554 only root can open sockets on privileged ports) and doesn't conflict with
555 any ports already in use. The connection is forwarded to port 6667 on
556 the remote server, since that's the standard port for IRC services.
557
558 The -f option backgrounds ssh and the remote command ``sleep 10'' is
559 specified to allow an amount of time (10 seconds, in the example) to
560 start the service which is to be tunnelled. If no connections are made
561 within the time specified, ssh will exit.
562
563X11 FORWARDING
564 If the ForwardX11 variable is set to ``yes'' (or see the description of
565 the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
566 environment variable is set), the connection to the X11 display is
567 automatically forwarded to the remote side in such a way that any X11
568 programs started from the shell (or command) will go through the
569 encrypted channel, and the connection to the real X server will be made
570 from the local machine. The user should not manually set DISPLAY.
571 Forwarding of X11 connections can be configured on the command line or in
572 configuration files.
573
574 The DISPLAY value set by ssh will point to the server machine, but with a
575 display number greater than zero. This is normal, and happens because
576 ssh creates a ``proxy'' X server on the server machine for forwarding the
577 connections over the encrypted channel.
578
579 ssh will also automatically set up Xauthority data on the server machine.
580 For this purpose, it will generate a random authorization cookie, store
581 it in Xauthority on the server, and verify that any forwarded connections
582 carry this cookie and replace it by the real cookie when the connection
583 is opened. The real authentication cookie is never sent to the server
584 machine (and no cookies are sent in the plain).
585
586 If the ForwardAgent variable is set to ``yes'' (or see the description of
587 the -A and -a options above) and the user is using an authentication
588 agent, the connection to the agent is automatically forwarded to the
589 remote side.
590
591VERIFYING HOST KEYS
592 When connecting to a server for the first time, a fingerprint of the
593 server's public key is presented to the user (unless the option
594 StrictHostKeyChecking has been disabled). Fingerprints can be determined
595 using ssh-keygen(1):
596
597 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
598
599 If the fingerprint is already known, it can be matched and the key can be
600 accepted or rejected. Because of the difficulty of comparing host keys
601 just by looking at hex strings, there is also support to compare host
602 keys visually, using random art. By setting the VisualHostKey option to
603 ``yes'', a small ASCII graphic gets displayed on every login to a server,
604 no matter if the session itself is interactive or not. By learning the
605 pattern a known server produces, a user can easily find out that the host
606 key has changed when a completely different pattern is displayed.
607 Because these patterns are not unambiguous however, a pattern that looks
608 similar to the pattern remembered only gives a good probability that the
609 host key is the same, not guaranteed proof.
610
611 To get a listing of the fingerprints along with their random art for all
612 known hosts, the following command line can be used:
613
614 $ ssh-keygen -lv -f ~/.ssh/known_hosts
615
616 If the fingerprint is unknown, an alternative method of verification is
617 available: SSH fingerprints verified by DNS. An additional resource
618 record (RR), SSHFP, is added to a zonefile and the connecting client is
619 able to match the fingerprint with that of the key presented.
620
621 In this example, we are connecting a client to a server,
622 ``host.example.com''. The SSHFP resource records should first be added
623 to the zonefile for host.example.com:
624
625 $ ssh-keygen -r host.example.com.
626
627 The output lines will have to be added to the zonefile. To check that
628 the zone is answering fingerprint queries:
629
630 $ dig -t SSHFP host.example.com
631
632 Finally the client connects:
633
634 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
635 [...]
636 Matching host key fingerprint found in DNS.
637 Are you sure you want to continue connecting (yes/no)?
638
639 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
640
641SSH-BASED VIRTUAL PRIVATE NETWORKS
642 ssh contains support for Virtual Private Network (VPN) tunnelling using
643 the tun(4) network pseudo-device, allowing two networks to be joined
644 securely. The sshd_config(5) configuration option PermitTunnel controls
645 whether the server supports this, and at what level (layer 2 or 3
646 traffic).
647
648 The following example would connect client network 10.0.50.0/24 with
649 remote network 10.0.99.0/24 using a point-to-point connection from
650 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway
651 to the remote network, at 192.168.1.15, allows it.
652
653 On the client:
654
655 # ssh -f -w 0:1 192.168.1.15 true
656 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
657 # route add 10.0.99.0/24 10.1.1.2
658
659 On the server:
660
661 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
662 # route add 10.0.50.0/24 10.1.1.1
663
664 Client access may be more finely tuned via the /root/.ssh/authorized_keys
665 file (see below) and the PermitRootLogin server option. The following
666 entry would permit connections on tun(4) device 1 from user ``jane'' and
667 on tun device 2 from user ``john'', if PermitRootLogin is set to
668 ``forced-commands-only'':
669
670 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
671 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
672
673 Since an SSH-based setup entails a fair amount of overhead, it may be
674 more suited to temporary setups, such as for wireless VPNs. More
675 permanent VPNs are better provided by tools such as ipsecctl(8) and
676 isakmpd(8).
677
678ENVIRONMENT
679 ssh will normally set the following environment variables:
680
681 DISPLAY The DISPLAY variable indicates the location of the
682 X11 server. It is automatically set by ssh to
683 point to a value of the form ``hostname:n'', where
684 ``hostname'' indicates the host where the shell
685 runs, and `n' is an integer >= 1. ssh uses this
686 special value to forward X11 connections over the
687 secure channel. The user should normally not set
688 DISPLAY explicitly, as that will render the X11
689 connection insecure (and will require the user to
690 manually copy any required authorization cookies).
691
692 HOME Set to the path of the user's home directory.
693
694 LOGNAME Synonym for USER; set for compatibility with
695 systems that use this variable.
696
697 MAIL Set to the path of the user's mailbox.
698
699 PATH Set to the default PATH, as specified when
700 compiling ssh.
701
702 SSH_ASKPASS If ssh needs a passphrase, it will read the
703 passphrase from the current terminal if it was run
704 from a terminal. If ssh does not have a terminal
705 associated with it but DISPLAY and SSH_ASKPASS are
706 set, it will execute the program specified by
707 SSH_ASKPASS and open an X11 window to read the
708 passphrase. This is particularly useful when
709 calling ssh from a .xsession or related script.
710 (Note that on some machines it may be necessary to
711 redirect the input from /dev/null to make this
712 work.)
713
714 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
715 communicate with the agent.
716
717 SSH_CONNECTION Identifies the client and server ends of the
718 connection. The variable contains four space-
719 separated values: client IP address, client port
720 number, server IP address, and server port number.
721
722 SSH_ORIGINAL_COMMAND This variable contains the original command line if
723 a forced command is executed. It can be used to
724 extract the original arguments.
725
726 SSH_TTY This is set to the name of the tty (path to the
727 device) associated with the current shell or
728 command. If the current session has no tty, this
729 variable is not set.
730
731 TZ This variable is set to indicate the present time
732 zone if it was set when the daemon was started
733 (i.e. the daemon passes the value on to new
734 connections).
735
736 USER Set to the name of the user logging in.
737
738 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
739 ``VARNAME=value'' to the environment if the file exists and users are
740 allowed to change their environment. For more information, see the
741 PermitUserEnvironment option in sshd_config(5).
742
743FILES
744 ~/.rhosts
745 This file is used for host-based authentication (see above). On
746 some machines this file may need to be world-readable if the
747 user's home directory is on an NFS partition, because sshd(8)
748 reads it as root. Additionally, this file must be owned by the
749 user, and must not have write permissions for anyone else. The
750 recommended permission for most machines is read/write for the
751 user, and not accessible by others.
752
753 ~/.shosts
754 This file is used in exactly the same way as .rhosts, but allows
755 host-based authentication without permitting login with
756 rlogin/rsh.
757
758 ~/.ssh/
759 This directory is the default location for all user-specific
760 configuration and authentication information. There is no
761 general requirement to keep the entire contents of this directory
762 secret, but the recommended permissions are read/write/execute
763 for the user, and not accessible by others.
764
765 ~/.ssh/authorized_keys
766 Lists the public keys (RSA/DSA) that can be used for logging in
767 as this user. The format of this file is described in the
768 sshd(8) manual page. This file is not highly sensitive, but the
769 recommended permissions are read/write for the user, and not
770 accessible by others.
771
772 ~/.ssh/config
773 This is the per-user configuration file. The file format and
774 configuration options are described in ssh_config(5). Because of
775 the potential for abuse, this file must have strict permissions:
776 read/write for the user, and not accessible by others.
777
778 ~/.ssh/environment
779 Contains additional definitions for environment variables; see
780 ENVIRONMENT, above.
781
782 ~/.ssh/identity
783 ~/.ssh/id_dsa
784 ~/.ssh/id_rsa
785 Contains the private key for authentication. These files contain
786 sensitive data and should be readable by the user but not
787 accessible by others (read/write/execute). ssh will simply
788 ignore a private key file if it is accessible by others. It is
789 possible to specify a passphrase when generating the key which
790 will be used to encrypt the sensitive part of this file using
791 3DES.
792
793 ~/.ssh/identity.pub
794 ~/.ssh/id_dsa.pub
795 ~/.ssh/id_rsa.pub
796 Contains the public key for authentication. These files are not
797 sensitive and can (but need not) be readable by anyone.
798
799 ~/.ssh/known_hosts
800 Contains a list of host keys for all hosts the user has logged
801 into that are not already in the systemwide list of known host
802 keys. See sshd(8) for further details of the format of this
803 file.
804
805 ~/.ssh/rc
806 Commands in this file are executed by ssh when the user logs in,
807 just before the user's shell (or command) is started. See the
808 sshd(8) manual page for more information.
809
810 /etc/hosts.equiv
811 This file is for host-based authentication (see above). It
812 should only be writable by root.
813
814 /etc/shosts.equiv
815 This file is used in exactly the same way as hosts.equiv, but
816 allows host-based authentication without permitting login with
817 rlogin/rsh.
818
819 /etc/ssh/ssh_config
820 Systemwide configuration file. The file format and configuration
821 options are described in ssh_config(5).
822
823 /etc/ssh/ssh_host_key
824 /etc/ssh/ssh_host_dsa_key
825 /etc/ssh/ssh_host_rsa_key
826 These three files contain the private parts of the host keys and
827 are used for host-based authentication. If protocol version 1 is
828 used, ssh must be setuid root, since the host key is readable
829 only by root. For protocol version 2, ssh uses ssh-keysign(8) to
830 access the host keys, eliminating the requirement that ssh be
831 setuid root when host-based authentication is used. By default
832 ssh is not setuid root.
833
834 /etc/ssh/ssh_known_hosts
835 Systemwide list of known host keys. This file should be prepared
836 by the system administrator to contain the public host keys of
837 all machines in the organization. It should be world-readable.
838 See sshd(8) for further details of the format of this file.
839
840 /etc/ssh/sshrc
841 Commands in this file are executed by ssh when the user logs in,
842 just before the user's shell (or command) is started. See the
843 sshd(8) manual page for more information.
844
845SEE ALSO
846 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
847 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8)
848
849 The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006.
850
851 The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006.
852
853 The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006.
854
855 The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.
856
857 The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.
858
859 Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC
860 4255, 2006.
861
862 Generic Message Exchange Authentication for the Secure Shell Protocol
863 (SSH), RFC 4256, 2006.
864
865 The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.
866
867 The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006.
868
869 Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer
870 Protocol, RFC 4345, 2006.
871
872 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer
873 Protocol, RFC 4419, 2006.
874
875 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
876
877 A. Perrig, D. Song, Hash Visualization: a New Technique to improve
878 Real-World Security, 1999, International Workshop on Cryptographic
879 Techniques and E-Commerce (CrypTEC '99).
880
881AUTHORS
882 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
883 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
884 de Raadt and Dug Song removed many bugs, re-added newer features and
885 created OpenSSH. Markus Friedl contributed the support for SSH protocol
886 versions 1.5 and 2.0.
887
888OpenBSD 4.8 August 4, 2010 OpenBSD 4.8