1SSH(1) BSD General Commands Manual SSH(1)
2
4 ssh — OpenSSH SSH client (remote login program)
5
7 ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec] [-D
8 [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] [-R
11 [bind_address:]port:host:hostport] [-S ctl_path] [-W host:port] [-w
12 local_tun[:remote_tun]] [user@]hostname [command]
13
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 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 If command is specified, it is executed on the remote host instead of a
27 login shell.
28 The options are as follows:
30 -1 Forces ssh to try protocol version 1 only.
32 -2 Forces ssh to try protocol version 2 only.
34 -4 Forces ssh to use IPv4 addresses only.
36 -6 Forces ssh to use IPv6 addresses only.
38 -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 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 -a Disables forwarding of the authentication agent connection.
52 -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 -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 pro‐
62 tocol 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 -c cipher_spec
68 Selects the cipher specification for encrypting the session.
69 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 dif‐
73 ferent 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 interoperabil‐
76 ity with legacy protocol 1 implementations that do not support
77 the 3des cipher. Its use is strongly discouraged due to crypto‐
78 graphic weaknesses. The default is “3des”.
79 For protocol version 2, cipher_spec is a comma-separated list of
81 ciphers listed in order of preference. See the Ciphers keyword
82 for more information.
83 -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 configura‐
94 tion file.
95 IPv6 addresses can be specified with an alternative syntax:
97 [bind_address/]port or by enclosing the address in square brack‐
98 ets. Only the superuser can forward privileged ports. By
99 default, the local port is bound in accordance with the
100 GatewayPorts setting. However, an explicit bind_address may be
101 used to bind the connection to a specific address. The
102 bind_address of “localhost” indicates that the listening port be
103 bound for local use only, while an empty address or ‘*’ indicates
104 that the port should be available from all interfaces.
105 -e escape_char
107 Sets the escape character for sessions with a pty (default: ‘~’).
108 The escape character is only recognized at the beginning of a
109 line. The escape character followed by a dot (‘.’) closes the
110 connection; followed by control-Z suspends the connection; and
111 followed by itself sends the escape character once. Setting the
112 character to “none” disables any escapes and makes the session
113 fully transparent.
114 -F configfile
116 Specifies an alternative per-user configuration file. If a con‐
117 figuration file is given on the command line, the system-wide
118 configuration file (/etc/ssh/ssh_config) will be ignored. The
119 default for the per-user configuration file is ~/.ssh/config.
120 -f Requests ssh to go to background just before command execution.
122 This is useful if ssh is going to ask for passwords or
123 passphrases, but the user wants it in the background. This
124 implies -n. The recommended way to start X11 programs at a
125 remote site is with something like ssh -f host xterm.
126 If the ExitOnForwardFailure configuration option is set to “yes”,
128 then a client started with -f will wait for all remote port for‐
129 wards to be successfully established before placing itself in the
130 background.
131 -g Allows remote hosts to connect to local forwarded ports.
133 -I pkcs11
135 Specify the PKCS#11 shared libarary ssh should use to communicate
136 with a PKCS#11 token used for storing the user's private RSA key.
137 This option is only available if support for PKCS#11 is compiled
138 in (default is no support).
139 -i identity_file
141 Selects a file from which the identity (private key) for public
142 key authentication is read. The default is ~/.ssh/identity for
143 protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa and
144 ~/.ssh/id_rsa for protocol version 2. Identity files may also be
145 specified on a per-host basis in the configuration file. It is
146 possible to have multiple -i options (and multiple identities
147 specified in configuration files). ssh will also try to load
148 certificate information from the filename obtained by appending
149 -cert.pub to identity filenames.
150 -K Enables GSSAPI-based authentication and forwarding (delegation)
152 of GSSAPI credentials to the server.
153 -k Disables forwarding (delegation) of GSSAPI credentials to the
155 server.
156 -L [bind_address:]port:host:hostport
158 Specifies that the given port on the local (client) host is to be
159 forwarded to the given host and port on the remote side. This
160 works by allocating a socket to listen to port on the local side,
161 optionally bound to the specified bind_address. Whenever a con‐
162 nection is made to this port, the connection is forwarded over
163 the secure channel, and a connection is made to host port
164 hostport from the remote machine. Port forwardings can also be
165 specified in the configuration file. IPv6 addresses can be spec‐
166 ified with an alternative syntax:
167 [bind_address/]port/host/hostport or by enclosing the address in
168 square brackets. Only the superuser can forward privileged
169 ports. By default, the local port is bound in accordance with
170 the GatewayPorts setting. However, an explicit bind_address may
171 be used to bind the connection to a specific address. The
172 bind_address of “localhost” indicates that the listening port be
173 bound for local use only, while an empty address or ‘*’ indicates
174 that the port should be available from all interfaces.
175 -l login_name
177 Specifies the user to log in as on the remote machine. This also
178 may be specified on a per-host basis in the configuration file.
179 -M Places the ssh client into “master” mode for connection sharing.
181 Multiple -M options places ssh into “master” mode with confirma‐
182 tion required before slave connections are accepted. Refer to
183 the description of ControlMaster in ssh_config(5) for details.
184 -m mac_spec
186 Additionally, for protocol version 2 a comma-separated list of
187 MAC (message authentication code) algorithms can be specified in
188 order of preference. See the MACs keyword for more information.
189 -N Do not execute a remote command. This is useful for just for‐
191 warding ports (protocol version 2 only).
192 -n Redirects stdin from /dev/null (actually, prevents reading from
194 stdin). This must be used when ssh is run in the background. A
195 common trick is to use this to run X11 programs on a remote
196 machine. For example, ssh -n shadows.cs.hut.fi emacs & will
197 start an emacs on shadows.cs.hut.fi, and the X11 connection will
198 be automatically forwarded over an encrypted channel. The ssh
199 program will be put in the background. (This does not work if
200 ssh needs to ask for a password or passphrase; see also the -f
201 option.)
202 -O ctl_cmd
204 Control an active connection multiplexing master process. When
205 the -O option is specified, the ctl_cmd argument is interpreted
206 and passed to the master process. Valid commands are: “check”
207 (check that the master process is running) and “exit” (request
208 the master to exit).
209 -o option
211 Can be used to give options in the format used in the configura‐
212 tion file. This is useful for specifying options for which there
213 is no separate command-line flag. For full details of the
214 options listed below, and their possible values, see
215 ssh_config(5).
216 AddressFamily
218 BatchMode
219 BindAddress
220 ChallengeResponseAuthentication
221 CheckHostIP
222 Cipher
223 Ciphers
224 ClearAllForwardings
225 Compression
226 CompressionLevel
227 ConnectionAttempts
228 ConnectTimeout
229 ControlMaster
230 ControlPath
231 ControlPersist
232 DynamicForward
233 EscapeChar
234 ExitOnForwardFailure
235 ForwardAgent
236 ForwardX11
237 ForwardX11Trusted
238 GatewayPorts
239 GlobalKnownHostsFile
240 GSSAPIAuthentication
241 GSSAPIKeyExchange
242 GSSAPIClientIdentity
243 GSSAPIDelegateCredentials
244 GSSAPIRenewalForcesRekey
245 GSSAPITrustDns
246 GSSAPIKexAlgorithms
247 HashKnownHosts
248 Host
249 HostbasedAuthentication
250 HostKeyAlgorithms
251 HostKeyAlias
252 HostName
253 IdentityFile
254 IdentitiesOnly
255 KbdInteractiveAuthentication
256 KbdInteractiveDevices
257 KexAlgorithms
258 LocalCommand
259 LocalForward
260 LogLevel
261 MACs
262 NoHostAuthenticationForLocalhost
263 NumberOfPasswordPrompts
264 PasswordAuthentication
265 PermitLocalCommand
266 PKCS11Provider
267 Port
268 PreferredAuthentications
269 Protocol
270 ProxyCommand
271 PubkeyAuthentication
272 RekeyLimit
273 RemoteForward
274 RhostsRSAAuthentication
275 RSAAuthentication
276 SendEnv
277 ServerAliveInterval
278 ServerAliveCountMax
279 StrictHostKeyChecking
280 TCPKeepAlive
281 Tunnel
282 TunnelDevice
283 UsePrivilegedPort
284 User
285 UserKnownHostsFile
286 VerifyHostKeyDNS
287 VisualHostKey
288 XAuthLocation
289 -p port
291 Port to connect to on the remote host. This can be specified on
292 a per-host basis in the configuration file.
293 -q Quiet mode. Causes most warning and diagnostic messages to be
295 suppressed.
296 -R [bind_address:]port:host:hostport
298 Specifies that the given port on the remote (server) host is to
299 be forwarded to the given host and port on the local side. This
300 works by allocating a socket to listen to port on the remote
301 side, and whenever a connection is made to this port, the connec‐
302 tion is forwarded over the secure channel, and a connection is
303 made to host port hostport from the local machine.
304 Port forwardings can also be specified in the configuration file.
306 Privileged ports can be forwarded only when logging in as root on
307 the remote machine. IPv6 addresses can be specified by enclosing
308 the address in square braces or using an alternative syntax:
309 [bind_address/]host/port/hostport.
310 By default, the listening socket on the server will be bound to
312 the loopback interface only. This may be overridden by specify‐
313 ing a bind_address. An empty bind_address, or the address ‘*’,
314 indicates that the remote socket should listen on all interfaces.
315 Specifying a remote bind_address will only succeed if the
316 server's GatewayPorts option is enabled (see sshd_config(5)).
317 If the port argument is ‘0’, the listen port will be dynamically
319 allocated on the server and reported to the client at run time.
320 -S ctl_path
322 Specifies the location of a control socket for connection shar‐
323 ing. Refer to the description of ControlPath and ControlMaster
324 in ssh_config(5) for details.
325 -s May be used to request invocation of a subsystem on the remote
327 system. Subsystems are a feature of the SSH2 protocol which
328 facilitate the use of SSH as a secure transport for other appli‐
329 cations (eg. sftp(1)). The subsystem is specified as the remote
330 command.
331 -T Disable pseudo-tty allocation.
333 -t Force pseudo-tty allocation. This can be used to execute arbi‐
335 trary screen-based programs on a remote machine, which can be
336 very useful, e.g. when implementing menu services. Multiple -t
337 options force tty allocation, even if ssh has no local tty.
338 -V Display the version number and exit.
340 -v Verbose mode. Causes ssh to print debugging messages about its
342 progress. This is helpful in debugging connection, authentica‐
343 tion, and configuration problems. Multiple -v options increase
344 the verbosity. The maximum is 3.
345 -W host:port
347 Requests that standard input and output on the client be for‐
348 warded to host on port over the secure channel. Implies -N, -T,
349 ExitOnForwardFailure and ClearAllForwardings and works with Pro‐
350 tocol version 2 only.
351 -w local_tun[:remote_tun]
353 Requests tunnel device forwarding with the specified tun(4)
354 devices between the client (local_tun) and the server
355 (remote_tun).
356 The devices may be specified by numerical ID or the keyword
358 “any”, which uses the next available tunnel device. If
359 remote_tun is not specified, it defaults to “any”. See also the
360 Tunnel and TunnelDevice directives in ssh_config(5). If the
361 Tunnel directive is unset, it is set to the default tunnel mode,
362 which is “point-to-point”.
363 -X Enables X11 forwarding. This can also be specified on a per-host
365 basis in a configuration file.
366 X11 forwarding should be enabled with caution. Users with the
368 ability to bypass file permissions on the remote host (for the
369 user's X authorization database) can access the local X11 display
370 through the forwarded connection. An attacker may then be able
371 to perform activities such as keystroke monitoring.
372 For this reason, X11 forwarding is subjected to X11 SECURITY
374 extension restrictions by default. Please refer to the ssh -Y
375 option and the ForwardX11Trusted directive in ssh_config(5) for
376 more information.
377 -x Disables X11 forwarding.
379 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
381 subjected to the X11 SECURITY extension controls.
382 -y Send log information using the syslog(3) system module. By
384 default this information is sent to stderr.
385 ssh may additionally obtain configuration data from a per-user configura‐
387 tion file and a system-wide configuration file. The file format and con‐
388 figuration options are described in ssh_config(5).
389 ssh exits with the exit status of the remote command or with 255 if an
391 error occurred.
392
394 The OpenSSH SSH client supports SSH protocols 1 and 2. Protocol 2 is the
395 default, with ssh falling back to protocol 1 if it detects protocol 2 is
396 unsupported. These settings may be altered using the Protocol option in
397 ssh_config(5), or enforced using the -1 and -2 options (see above). Both
398 protocols support similar authentication methods, but protocol 2 is pre‐
399 ferred since it provides additional mechanisms for confidentiality (the
400 traffic is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and
401 integrity (hmac-md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512, umac-64,
402 hmac-ripemd160). Protocol 1 lacks a strong mechanism for ensuring the
403 integrity of the connection.
404 The methods available for authentication are: GSSAPI-based authentica‐
406 tion, host-based authentication, public key authentication, challenge-
407 response authentication, and password authentication. Authentication
408 methods are tried in the order specified above, though protocol 2 has a
409 configuration option to change the default order:
410 PreferredAuthentications.
411 Host-based authentication works as follows: If the machine the user logs
413 in from is listed in /etc/hosts.equiv or /etc/ssh/shosts.equiv on the
414 remote machine, and the user names are the same on both sides, or if the
415 files ~/.rhosts or ~/.shosts exist in the user's home directory on the
416 remote machine and contain a line containing the name of the client
417 machine and the name of the user on that machine, the user is considered
418 for login. Additionally, the server must be able to verify the client's
419 host key (see the description of /etc/ssh/ssh_known_hosts and
420 ~/.ssh/known_hosts, below) for login to be permitted. This authentica‐
421 tion method closes security holes due to IP spoofing, DNS spoofing, and
422 routing spoofing. [Note to the administrator: /etc/hosts.equiv,
423 ~/.rhosts, and the rlogin/rsh protocol in general, are inherently inse‐
424 cure and should be disabled if security is desired.]
425 Public key authentication works as follows: The scheme is based on pub‐
427 lic-key cryptography, using cryptosystems where encryption and decryption
428 are done using separate keys, and it is unfeasible to derive the decryp‐
429 tion key from the encryption key. The idea is that each user creates a
430 public/private key pair for authentication purposes. The server knows
431 the public key, and only the user knows the private key. ssh implements
432 public key authentication protocol automatically, using one of the DSA,
433 ECDSA or RSA algorithms. Protocol 1 is restricted to using only RSA
434 keys, but protocol 2 may use any. The HISTORY section of ssl(8) contains
435 a brief discussion of the two algorithms.
436 The file ~/.ssh/authorized_keys lists the public keys that are permitted
438 for logging in. When the user logs in, the ssh program tells the server
439 which key pair it would like to use for authentication. The client
440 proves that it has access to the private key and the server checks that
441 the corresponding public key is authorized to accept the account.
442 The user creates his/her key pair by running ssh-keygen(1). This stores
444 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol
445 2 DSA), ~/.ssh/id_ecdsa (protocol 2 ECDSA), or ~/.ssh/id_rsa (protocol 2
446 RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1),
447 ~/.ssh/id_dsa.pub (protocol 2 DSA), ~/.ssh/id_ecdsa.pub (protocol 2
448 ECDSA), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home direc‐
449 tory. The user should then copy the public key to ~/.ssh/authorized_keys
450 in his/her home directory on the remote machine. The authorized_keys
451 file corresponds to the conventional ~/.rhosts file, and has one key per
452 line, though the lines can be very long. After this, the user can log in
453 without giving the password.
454 A variation on public key authentication is available in the form of cer‐
456 tificate authentication: instead of a set of public/private keys, signed
457 certificates are used. This has the advantage that a single trusted cer‐
458 tification authority can be used in place of many public/private keys.
459 See the CERTIFICATES section of ssh-keygen(1) for more information.
460 The most convenient way to use public key or certificate authentication
462 may be with an authentication agent. See ssh-agent(1) for more informa‐
463 tion.
464 Challenge-response authentication works as follows: The server sends an
466 arbitrary "challenge" text, and prompts for a response. Protocol 2
467 allows multiple challenges and responses; protocol 1 is restricted to
468 just one challenge/response. Examples of challenge-response authentica‐
469 tion include BSD Authentication (see login.conf(5)) and PAM (some non-
470 OpenBSD systems).
471 Finally, if other authentication methods fail, ssh prompts the user for a
473 password. The password is sent to the remote host for checking; however,
474 since all communications are encrypted, the password cannot be seen by
475 someone listening on the network.
476 ssh automatically maintains and checks a database containing identifica‐
478 tion for all hosts it has ever been used with. Host keys are stored in
479 ~/.ssh/known_hosts in the user's home directory. Additionally, the file
480 /etc/ssh/ssh_known_hosts is automatically checked for known hosts. Any
481 new hosts are automatically added to the user's file. If a host's iden‐
482 tification ever changes, ssh warns about this and disables password
483 authentication to prevent server spoofing or man-in-the-middle attacks,
484 which could otherwise be used to circumvent the encryption. The
485 StrictHostKeyChecking option can be used to control logins to machines
486 whose host key is not known or has changed.
487 When the user's identity has been accepted by the server, the server
489 either executes the given command, or logs into the machine and gives the
490 user a normal shell on the remote machine. All communication with the
491 remote command or shell will be automatically encrypted.
492 If a pseudo-terminal has been allocated (normal login session), the user
494 may use the escape characters noted below.
495 If no pseudo-tty has been allocated, the session is transparent and can
497 be used to reliably transfer binary data. On most systems, setting the
498 escape character to “none” will also make the session transparent even if
499 a tty is used.
500 The session terminates when the command or shell on the remote machine
502 exits and all X11 and TCP connections have been closed.
503
505 When a pseudo-terminal has been requested, ssh supports a number of func‐
506 tions through the use of an escape character.
507 A single tilde character can be sent as ~~ or by following the tilde by a
509 character other than those described below. The escape character must
510 always follow a newline to be interpreted as special. The escape charac‐
511 ter can be changed in configuration files using the EscapeChar configura‐
512 tion directive or on the command line by the -e option.
513 The supported escapes (assuming the default ‘~’) are:
515 ~. Disconnect.
517 ~^Z Background ssh.
519 ~# List forwarded connections.
521 ~& Background ssh at logout when waiting for forwarded connection /
523 X11 sessions to terminate.
524 ~? Display a list of escape characters.
526 ~B Send a BREAK to the remote system (only useful for SSH protocol
528 version 2 and if the peer supports it).
529 ~C Open command line. Currently this allows the addition of port
531 forwardings using the -L, -R and -D options (see above). It also
532 allows the cancellation of existing remote port-forwardings using
533 -KR[bind_address:]port. !command allows the user to execute a
534 local command if the PermitLocalCommand option is enabled in
535 ssh_config(5). Basic help is available, using the -h option.
536 ~R Request rekeying of the connection (only useful for SSH protocol
538 version 2 and if the peer supports it).
539
541 Forwarding of arbitrary TCP connections over the secure channel can be
542 specified either on the command line or in a configuration file. One
543 possible application of TCP forwarding is a secure connection to a mail
544 server; another is going through firewalls.
545 In the example below, we look at encrypting communication between an IRC
547 client and server, even though the IRC server does not directly support
548 encrypted communications. This works as follows: the user connects to
549 the remote host using ssh, specifying a port to be used to forward con‐
550 nections to the remote server. After that it is possible to start the
551 service which is to be encrypted on the client machine, connecting to the
552 same local port, and ssh will encrypt and forward the connection.
553 The following example tunnels an IRC session from client machine
555 “127.0.0.1” (localhost) to remote server “server.example.com”:
556 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
558 $ irc -c '#users' -p 1234 pinky 127.0.0.1
559 This tunnels a connection to IRC server “server.example.com”, joining
561 channel “#users”, nickname “pinky”, using port 1234. It doesn't matter
562 which port is used, as long as it's greater than 1023 (remember, only
563 root can open sockets on privileged ports) and doesn't conflict with any
564 ports already in use. The connection is forwarded to port 6667 on the
565 remote server, since that's the standard port for IRC services.
566 The -f option backgrounds ssh and the remote command “sleep 10” is speci‐
568 fied to allow an amount of time (10 seconds, in the example) to start the
569 service which is to be tunnelled. If no connections are made within the
570 time specified, ssh will exit.
571
573 If the ForwardX11 variable is set to “yes” (or see the description of the
574 -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
575 environment variable is set), the connection to the X11 display is auto‐
576 matically forwarded to the remote side in such a way that any X11 pro‐
577 grams started from the shell (or command) will go through the encrypted
578 channel, and the connection to the real X server will be made from the
579 local machine. The user should not manually set DISPLAY. Forwarding of
580 X11 connections can be configured on the command line or in configuration
581 files.
582 The DISPLAY value set by ssh will point to the server machine, but with a
584 display number greater than zero. This is normal, and happens because
585 ssh creates a “proxy” X server on the server machine for forwarding the
586 connections over the encrypted channel.
587 ssh will also automatically set up Xauthority data on the server machine.
589 For this purpose, it will generate a random authorization cookie, store
590 it in Xauthority on the server, and verify that any forwarded connections
591 carry this cookie and replace it by the real cookie when the connection
592 is opened. The real authentication cookie is never sent to the server
593 machine (and no cookies are sent in the plain).
594 If the ForwardAgent variable is set to “yes” (or see the description of
596 the -A and -a options above) and the user is using an authentication
597 agent, the connection to the agent is automatically forwarded to the
598 remote side.
599
601 When connecting to a server for the first time, a fingerprint of the
602 server's public key is presented to the user (unless the option
603 StrictHostKeyChecking has been disabled). Fingerprints can be determined
604 using ssh-keygen(1):
605 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
607 If the fingerprint is already known, it can be matched and the key can be
609 accepted or rejected. Because of the difficulty of comparing host keys
610 just by looking at hex strings, there is also support to compare host
611 keys visually, using random art. By setting the VisualHostKey option to
612 “yes”, a small ASCII graphic gets displayed on every login to a server,
613 no matter if the session itself is interactive or not. By learning the
614 pattern a known server produces, a user can easily find out that the host
615 key has changed when a completely different pattern is displayed.
616 Because these patterns are not unambiguous however, a pattern that looks
617 similar to the pattern remembered only gives a good probability that the
618 host key is the same, not guaranteed proof.
619 To get a listing of the fingerprints along with their random art for all
621 known hosts, the following command line can be used:
622 $ ssh-keygen -lv -f ~/.ssh/known_hosts
624 If the fingerprint is unknown, an alternative method of verification is
626 available: SSH fingerprints verified by DNS. An additional resource
627 record (RR), SSHFP, is added to a zonefile and the connecting client is
628 able to match the fingerprint with that of the key presented.
629 In this example, we are connecting a client to a server,
631 “host.example.com”. The SSHFP resource records should first be added to
632 the zonefile for host.example.com:
633 $ ssh-keygen -r host.example.com.
635 The output lines will have to be added to the zonefile. To check that
637 the zone is answering fingerprint queries:
638 $ dig -t SSHFP host.example.com
640 Finally the client connects:
642 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
644 [...]
645 Matching host key fingerprint found in DNS.
646 Are you sure you want to continue connecting (yes/no)?
647 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
649
651 ssh contains support for Virtual Private Network (VPN) tunnelling using
652 the tun(4) network pseudo-device, allowing two networks to be joined
653 securely. The sshd_config(5) configuration option PermitTunnel controls
654 whether the server supports this, and at what level (layer 2 or 3 traf‐
655 fic).
656 The following example would connect client network 10.0.50.0/24 with
658 remote network 10.0.99.0/24 using a point-to-point connection from
659 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway
660 to the remote network, at 192.168.1.15, allows it.
661 On the client:
663 # ssh -f -w 0:1 192.168.1.15 true
665 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
666 # route add 10.0.99.0/24 10.1.1.2
667 On the server:
669 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
671 # route add 10.0.50.0/24 10.1.1.1
672 Client access may be more finely tuned via the /root/.ssh/authorized_keys
674 file (see below) and the PermitRootLogin server option. The following
675 entry would permit connections on tun(4) device 1 from user “jane” and on
676 tun device 2 from user “john”, if PermitRootLogin is set to
677 “forced-commands-only”:
678 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
680 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
681 Since an SSH-based setup entails a fair amount of overhead, it may be
683 more suited to temporary setups, such as for wireless VPNs. More perma‐
684 nent VPNs are better provided by tools such as ipsecctl(8) and
685 isakmpd(8).
686
688 ssh will normally set the following environment variables:
689 DISPLAY The DISPLAY variable indicates the location of the
691 X11 server. It is automatically set by ssh to
692 point to a value of the form “hostname:n”, where
693 “hostname” indicates the host where the shell runs,
694 and ‘n’ is an integer ≥ 1. ssh uses this special
695 value to forward X11 connections over the secure
696 channel. The user should normally not set DISPLAY
697 explicitly, as that will render the X11 connection
698 insecure (and will require the user to manually
699 copy any required authorization cookies).
700 HOME Set to the path of the user's home directory.
702 LOGNAME Synonym for USER; set for compatibility with sys‐
704 tems that use this variable.
705 MAIL Set to the path of the user's mailbox.
707 PATH Set to the default PATH, as specified when compil‐
709 ing ssh.
710 SSH_ASKPASS If ssh needs a passphrase, it will read the
712 passphrase from the current terminal if it was run
713 from a terminal. If ssh does not have a terminal
714 associated with it but DISPLAY and SSH_ASKPASS are
715 set, it will execute the program specified by
716 SSH_ASKPASS and open an X11 window to read the
717 passphrase. This is particularly useful when call‐
718 ing ssh from a .xsession or related script. (Note
719 that on some machines it may be necessary to redi‐
720 rect the input from /dev/null to make this work.)
721 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
723 communicate with the agent.
724 SSH_CONNECTION Identifies the client and server ends of the con‐
726 nection. The variable contains four space-sepa‐
727 rated values: client IP address, client port num‐
728 ber, server IP address, and server port number.
729 SSH_ORIGINAL_COMMAND This variable contains the original command line if
731 a forced command is executed. It can be used to
732 extract the original arguments.
733 SSH_TTY This is set to the name of the tty (path to the
735 device) associated with the current shell or com‐
736 mand. If the current session has no tty, this
737 variable is not set.
738 TZ This variable is set to indicate the present time
740 zone if it was set when the daemon was started
741 (i.e. the daemon passes the value on to new connec‐
742 tions).
743 USER Set to the name of the user logging in.
745 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
747 “VARNAME=value” to the environment if the file exists and users are
748 allowed to change their environment. For more information, see the
749 PermitUserEnvironment option in sshd_config(5).
750
752 SSH_USE_STRONG_RNG
753 The reseeding of the OpenSSL random generator is usually done
754 from /dev/urandom. If the SSH_USE_STRONG_RNG environment vari‐
755 able is set to value other than 0 the OpenSSL random generator is
756 reseeded from /dev/random. The number of bytes read is defined
757 by the SSH_USE_STRONG_RNG value. Minimum is 14 bytes. This set‐
758 ting is not recommended on the computers without the hardware
759 random generator because insufficient entropy causes the connec‐
760 tion to be blocked until enough entropy is available.
761
763 ~/.rhosts
764 This file is used for host-based authentication (see above). On
765 some machines this file may need to be world-readable if the
766 user's home directory is on an NFS partition, because sshd(8)
767 reads it as root. Additionally, this file must be owned by the
768 user, and must not have write permissions for anyone else. The
769 recommended permission for most machines is read/write for the
770 user, and not accessible by others.
771 ~/.shosts
773 This file is used in exactly the same way as .rhosts, but allows
774 host-based authentication without permitting login with
775 rlogin/rsh.
776 ~/.ssh/
778 This directory is the default location for all user-specific con‐
779 figuration and authentication information. There is no general
780 requirement to keep the entire contents of this directory secret,
781 but the recommended permissions are read/write/execute for the
782 user, and not accessible by others.
783 ~/.ssh/authorized_keys
785 Lists the public keys (RSA/ECDSA/DSA) that can be used for log‐
786 ging in as this user. The format of this file is described in
787 the sshd(8) manual page. This file is not highly sensitive, but
788 the recommended permissions are read/write for the user, and not
789 accessible by others.
790 ~/.ssh/config
792 This is the per-user configuration file. The file format and
793 configuration options are described in ssh_config(5). Because of
794 the potential for abuse, this file must have strict permissions:
795 read/write for the user, and not accessible by others.
796 ~/.ssh/environment
798 Contains additional definitions for environment variables; see
799 ENVIRONMENT, above.
800 ~/.ssh/identity
802 ~/.ssh/id_dsa
803 ~/.ssh/id_ecdsa
804 ~/.ssh/id_rsa
805 Contains the private key for authentication. These files contain
806 sensitive data and should be readable by the user but not acces‐
807 sible by others (read/write/execute). ssh will simply ignore a
808 private key file if it is accessible by others. It is possible
809 to specify a passphrase when generating the key which will be
810 used to encrypt the sensitive part of this file using 3DES.
811 ~/.ssh/identity.pub
813 ~/.ssh/id_dsa.pub
814 ~/.ssh/id_ecdsa.pub
815 ~/.ssh/id_rsa.pub
816 Contains the public key for authentication. These files are not
817 sensitive and can (but need not) be readable by anyone.
818 ~/.ssh/known_hosts
820 Contains a list of host keys for all hosts the user has logged
821 into that are not already in the systemwide list of known host
822 keys. See sshd(8) for further details of the format of this
823 file.
824 ~/.ssh/rc
826 Commands in this file are executed by ssh when the user logs in,
827 just before the user's shell (or command) is started. See the
828 sshd(8) manual page for more information.
829 /etc/hosts.equiv
831 This file is for host-based authentication (see above). It
832 should only be writable by root.
833 /etc/ssh/shosts.equiv
835 This file is used in exactly the same way as hosts.equiv, but
836 allows host-based authentication without permitting login with
837 rlogin/rsh.
838 /etc/ssh/ssh_config
840 Systemwide configuration file. The file format and configuration
841 options are described in ssh_config(5).
842 /etc/ssh/ssh_host_key
844 /etc/ssh/ssh_host_dsa_key
845 /etc/ssh/ssh_host_ecdsa_key
846 /etc/ssh/ssh_host_rsa_key
847 These three files contain the private parts of the host keys and
848 are used for host-based authentication. If protocol version 1 is
849 used, ssh must be setuid root, since the host key is readable
850 only by root. For protocol version 2, ssh uses ssh-keysign(8) to
851 access the host keys, eliminating the requirement that ssh be
852 setuid root when host-based authentication is used. By default
853 ssh is not setuid root.
854 /etc/ssh/ssh_known_hosts
856 Systemwide list of known host keys. This file should be prepared
857 by the system administrator to contain the public host keys of
858 all machines in the organization. It should be world-readable.
859 See sshd(8) for further details of the format of this file.
860 /etc/ssh/sshrc
862 Commands in this file are executed by ssh when the user logs in,
863 just before the user's shell (or command) is started. See the
864 sshd(8) manual page for more information.
865
867 IPv6 address can be used everywhere where IPv4 address. In all entries
868 must be the IPv6 address enclosed in square brackets. Note: The square
869 brackets are metacharacters for the shell and must be escaped in shell.
870
872 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
873 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8)
874 The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006.
876 The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006.
878 The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006.
880 The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.
882 The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.
884 Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC
886 4255, 2006.
887 Generic Message Exchange Authentication for the Secure Shell Protocol
889 (SSH), RFC 4256, 2006.
890 The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.
892 The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006.
894 Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer
896 Protocol, RFC 4345, 2006.
897 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer
899 Protocol, RFC 4419, 2006.
900 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
902 Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer,
904 RFC 5656, 2009.
905 A. Perrig and D. Song, Hash Visualization: a New Technique to improve
907 Real-World Security, 1999, International Workshop on Cryptographic
908 Techniques and E-Commerce (CrypTEC '99).
909
911 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
912 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
913 de Raadt and Dug Song removed many bugs, re-added newer features and cre‐
914 ated OpenSSH. Markus Friedl contributed the support for SSH protocol
915 versions 1.5 and 2.0.
916BSD June 21, 2019 BSD