1 SSH(1) General Commands Manual SSH(1)
4 ssh M-bM-^@M-^S OpenSSH SSH client (remote login program)
7 ssh [-1246AaCfGgKkMNnqsTtVvXxYy] [-b bind_address] [-c cipher_spec]
8 [-D [bind_address:]port] [-E log_file] [-e escape_char]
9 [-F configfile] [-I pkcs11] [-i identity_file]
10 [-L [bind_address:]port:host:hostport] [-l login_name] [-m mac_spec]
11 [-O ctl_cmd] [-o option] [-p port]
12 [-Q cipher | cipher-auth | mac | kex | key | protocol-version]
13 [-R [bind_address:]port:host:hostport] [-S ctl_path] [-W host:port]
14 [-w local_tun[:remote_tun]] [user@]hostname [command]
17 ssh (SSH client) is a program for logging into a remote machine and for
18 executing commands on a remote machine. It is intended to replace rlogin
19 and rsh, and provide secure encrypted communications between two
20 untrusted hosts over an insecure network. X11 connections, arbitrary TCP
21 ports and UNIX-domain sockets can also be forwarded over the secure
24 ssh connects and logs into the specified hostname (with optional user
25 name). The user must prove his/her identity to the remote machine using
26 one of several methods depending on the protocol version used (see
29 If command is specified, it is executed on the remote host instead of a
32 The options are as follows:
34 -1 Forces ssh to try protocol version 1 only.
36 -2 Forces ssh to try protocol version 2 only.
38 -4 Forces ssh to use IPv4 addresses only.
40 -6 Forces ssh to use IPv6 addresses only.
42 -A Enables forwarding of the authentication agent connection. This
43 can also be specified on a per-host basis in a configuration
46 Agent forwarding should be enabled with caution. Users with the
47 ability to bypass file permissions on the remote host (for the
48 agent's UNIX-domain socket) can access the local agent through
49 the forwarded connection. An attacker cannot obtain key material
50 from the agent, however they can perform operations on the keys
51 that enable them to authenticate using the identities loaded into
54 -a Disables forwarding of the authentication agent connection.
57 Use bind_address on the local machine as the source address of
58 the connection. Only useful on systems with more than one
61 -C Requests compression of all data (including stdin, stdout,
62 stderr, and data for forwarded X11, TCP and UNIX-domain
63 connections). The compression algorithm is the same used by
64 gzip(1), and the M-bM-^@M-^\levelM-bM-^@M-^] can be controlled by the
65 CompressionLevel option for protocol version 1. Compression is
66 desirable on modem lines and other slow connections, but will
67 only slow down things on fast networks. The default value can be
68 set on a host-by-host basis in the configuration files; see the
72 Selects the cipher specification for encrypting the session.
74 Protocol version 1 allows specification of a single cipher. The
75 supported values are M-bM-^@M-^\3desM-bM-^@M-^], M-bM-^@M-^\blowfishM-bM-^@M-^], and M-bM-^@M-^\desM-bM-^@M-^]. For protocol
76 version 2, cipher_spec is a comma-separated list of ciphers
77 listed in order of preference. See the Ciphers keyword in
78 ssh_config(5) for more information.
80 -D [bind_address:]port
81 Specifies a local M-bM-^@M-^\dynamicM-bM-^@M-^] application-level port forwarding.
82 This works by allocating a socket to listen to port on the local
83 side, optionally bound to the specified bind_address. Whenever a
84 connection is made to this port, the connection is forwarded over
85 the secure channel, and the application protocol is then used to
86 determine where to connect to from the remote machine. Currently
87 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
88 as a SOCKS server. Only root can forward privileged ports.
89 Dynamic port forwardings can also be specified in the
92 IPv6 addresses can be specified by enclosing the address in
93 square brackets. Only the superuser can forward privileged
94 ports. By default, the local port is bound in accordance with
95 the GatewayPorts setting. However, an explicit bind_address may
96 be used to bind the connection to a specific address. The
97 bind_address of M-bM-^@M-^\localhostM-bM-^@M-^] indicates that the listening port be
98 bound for local use only, while an empty address or M-bM-^@M-^X*M-bM-^@M-^Y indicates
99 that the port should be available from all interfaces.
102 Append debug logs to log_file instead of standard error.
105 Sets the escape character for sessions with a pty (default: M-bM-^@M-^X~M-bM-^@M-^Y).
106 The escape character is only recognized at the beginning of a
107 line. The escape character followed by a dot (M-bM-^@M-^X.M-bM-^@M-^Y) closes the
108 connection; followed by control-Z suspends the connection; and
109 followed by itself sends the escape character once. Setting the
110 character to M-bM-^@M-^\noneM-bM-^@M-^] disables any escapes and makes the session
114 Specifies an alternative per-user configuration file. If a
115 configuration file is given on the command line, the system-wide
116 configuration file (/etc/ssh/ssh_config) will be ignored. The
117 default for the per-user configuration file is ~/.ssh/config.
119 -f Requests ssh to go to background just before command execution.
120 This is useful if ssh is going to ask for passwords or
121 passphrases, but the user wants it in the background. This
122 implies -n. The recommended way to start X11 programs at a
123 remote site is with something like ssh -f host xterm.
125 If the ExitOnForwardFailure configuration option is set to M-bM-^@M-^\yesM-bM-^@M-^],
126 then a client started with -f will wait for all remote port
127 forwards to be successfully established before placing itself in
130 -G Causes ssh to print its configuration after evaluating Host and
131 Match blocks and exit.
133 -g Allows remote hosts to connect to local forwarded ports. If used
134 on a multiplexed connection, then this option must be specified
135 on the master process.
138 Specify the PKCS#11 shared library ssh should use to communicate
139 with a PKCS#11 token providing the user's private RSA key.
142 Selects a file from which the identity (private key) for public
143 key authentication is read. The default is ~/.ssh/identity for
144 protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa,
145 ~/.ssh/id_ed25519 and ~/.ssh/id_rsa for protocol version 2.
146 Identity files may also be specified on a per-host basis in the
147 configuration file. It is possible to have multiple -i options
148 (and multiple identities specified in configuration files). ssh
149 will also try to load certificate information from the filename
150 obtained by appending -cert.pub to identity filenames.
152 -K Enables GSSAPI-based authentication and forwarding (delegation)
153 of GSSAPI credentials to the server.
155 -k Disables forwarding (delegation) of GSSAPI credentials to the
158 -L [bind_address:]port:host:hostport
159 Specifies that the given port on the local (client) host is to be
160 forwarded to the given host and port on the remote side. This
161 works by allocating a socket to listen to port on the local side,
162 optionally bound to the specified bind_address. Whenever a
163 connection is made to this port, the connection is forwarded over
164 the secure channel, and a connection is made to host port
165 hostport from the remote machine. Port forwardings can also be
166 specified in the configuration file. IPv6 addresses can be
167 specified by enclosing the address in square brackets. Only the
168 superuser can forward privileged ports. By default, the local
169 port is bound in accordance with the GatewayPorts setting.
170 However, an explicit bind_address may be used to bind the
171 connection to a specific address. The bind_address of
172 M-bM-^@M-^\localhostM-bM-^@M-^] indicates that the listening port be bound for local
173 use only, while an empty address or M-bM-^@M-^X*M-bM-^@M-^Y indicates that the port
174 should be available from all interfaces.
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.
180 -M Places the ssh client into M-bM-^@M-^\masterM-bM-^@M-^] mode for connection sharing.
181 Multiple -M options places ssh into M-bM-^@M-^\masterM-bM-^@M-^] mode with
182 confirmation required before slave connections are accepted.
183 Refer to the description of ControlMaster in ssh_config(5) for
187 Additionally, for protocol version 2 a comma-separated list of
188 MAC (message authentication code) algorithms can be specified in
189 order of preference. See the MACs keyword for more information.
191 -N Do not execute a remote command. This is useful for just
192 forwarding ports (protocol version 2 only).
194 -n Redirects stdin from /dev/null (actually, prevents reading from
195 stdin). This must be used when ssh is run in the background. A
196 common trick is to use this to run X11 programs on a remote
197 machine. For example, ssh -n shadows.cs.hut.fi emacs & will
198 start an emacs on shadows.cs.hut.fi, and the X11 connection will
199 be automatically forwarded over an encrypted channel. The ssh
200 program will be put in the background. (This does not work if
201 ssh needs to ask for a password or passphrase; see also the -f
205 Control an active connection multiplexing master process. When
206 the -O option is specified, the ctl_cmd argument is interpreted
207 and passed to the master process. Valid commands are: M-bM-^@M-^\checkM-bM-^@M-^]
208 (check that the master process is running), M-bM-^@M-^\forwardM-bM-^@M-^] (request
209 forwardings without command execution), M-bM-^@M-^\cancelM-bM-^@M-^] (cancel
210 forwardings), M-bM-^@M-^\exitM-bM-^@M-^] (request the master to exit), and M-bM-^@M-^\stopM-bM-^@M-^]
211 (request the master to stop accepting further multiplexing
215 Can be used to give options in the format used in the
216 configuration file. This is useful for specifying options for
217 which there is no separate command-line flag. For full details
218 of the options listed below, and their possible values, see
225 CanonicalizeFallbackLocal
228 CanonicalizePermittedCNAMEs
229 ChallengeResponseAuthentication
252 GSSAPIDelegateCredentials
255 HostbasedAuthentication
263 KbdInteractiveAuthentication
264 KbdInteractiveDevices
271 NoHostAuthenticationForLocalhost
272 NumberOfPasswordPrompts
273 PasswordAuthentication
277 PreferredAuthentications
285 RhostsRSAAuthentication
291 StreamLocalBindUnlink
292 StrictHostKeyChecking
305 Port to connect to on the remote host. This can be specified on
306 a per-host basis in the configuration file.
308 -Q cipher | cipher-auth | mac | kex | key | protocol-version
309 Queries ssh for the algorithms supported for the specified
310 version 2. The available features are: cipher (supported
311 symmetric ciphers), cipher-auth (supported symmetric ciphers that
312 support authenticated encryption), mac (supported message
313 integrity codes), kex (key exchange algorithms), key (key types)
314 and protocol-version (supported SSH protocol versions).
316 -q Quiet mode. Causes most warning and diagnostic messages to be
319 -R [bind_address:]port:host:hostport
320 Specifies that the given port on the remote (server) host is to
321 be forwarded to the given host and port on the local side. This
322 works by allocating a socket to listen to port on the remote
323 side, and whenever a connection is made to this port, the
324 connection is forwarded over the secure channel, and a connection
325 is made to host port hostport from the local machine.
327 Port forwardings can also be specified in the configuration file.
328 Privileged ports can be forwarded only when logging in as root on
329 the remote machine. IPv6 addresses can be specified by enclosing
330 the address in square brackets.
332 By default, the listening socket on the server will be bound to
333 the loopback interface only. This may be overridden by
334 specifying a bind_address. An empty bind_address, or the address
335 M-bM-^@M-^X*M-bM-^@M-^Y, indicates that the remote socket should listen on all
336 interfaces. Specifying a remote bind_address will only succeed
337 if the server's GatewayPorts option is enabled (see
340 If the port argument is M-bM-^@M-^X0M-bM-^@M-^Y, the listen port will be dynamically
341 allocated on the server and reported to the client at run time.
342 When used together with -O forward the allocated port will be
343 printed to the standard output.
346 Specifies the location of a control socket for connection
347 sharing, or the string M-bM-^@M-^\noneM-bM-^@M-^] to disable connection sharing.
348 Refer to the description of ControlPath and ControlMaster in
349 ssh_config(5) for details.
351 -s May be used to request invocation of a subsystem on the remote
352 system. Subsystems are a feature of the SSH2 protocol which
353 facilitate the use of SSH as a secure transport for other
354 applications (eg. sftp(1)). The subsystem is specified as the
357 -T Disable pseudo-tty allocation.
359 -t Force pseudo-tty allocation. This can be used to execute
360 arbitrary screen-based programs on a remote machine, which can be
361 very useful, e.g. when implementing menu services. Multiple -t
362 options force tty allocation, even if ssh has no local tty.
364 -V Display the version number and exit.
366 -v Verbose mode. Causes ssh to print debugging messages about its
367 progress. This is helpful in debugging connection,
368 authentication, and configuration problems. Multiple -v options
369 increase the verbosity. The maximum is 3.
372 Requests that standard input and output on the client be
373 forwarded to host on port over the secure channel. Implies -N,
374 -T, ExitOnForwardFailure and ClearAllForwardings. Works with
375 Protocol version 2 only.
377 -w local_tun[:remote_tun]
378 Requests tunnel device forwarding with the specified tun(4)
379 devices between the client (local_tun) and the server
382 The devices may be specified by numerical ID or the keyword
383 M-bM-^@M-^\anyM-bM-^@M-^], which uses the next available tunnel device. If
384 remote_tun is not specified, it defaults to M-bM-^@M-^\anyM-bM-^@M-^]. See also the
385 Tunnel and TunnelDevice directives in ssh_config(5). If the
386 Tunnel directive is unset, it is set to the default tunnel mode,
387 which is M-bM-^@M-^\point-to-pointM-bM-^@M-^].
389 -X Enables X11 forwarding. This can also be specified on a per-host
390 basis in a configuration file.
392 X11 forwarding should be enabled with caution. Users with the
393 ability to bypass file permissions on the remote host (for the
394 user's X authorization database) can access the local X11 display
395 through the forwarded connection. An attacker may then be able
396 to perform activities such as keystroke monitoring.
398 For this reason, X11 forwarding is subjected to X11 SECURITY
399 extension restrictions by default. Please refer to the ssh -Y
400 option and the ForwardX11Trusted directive in ssh_config(5) for
403 -x Disables X11 forwarding.
405 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
406 subjected to the X11 SECURITY extension controls.
408 -y Send log information using the syslog(3) system module. By
409 default this information is sent to stderr.
411 ssh may additionally obtain configuration data from a per-user
412 configuration file and a system-wide configuration file. The file format
413 and configuration options are described in ssh_config(5).
416 The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to
417 use protocol 2 only, though this can be changed via the Protocol option
418 in ssh_config(5) or the -1 and -2 options (see above). Both protocols
419 support similar authentication methods, but protocol 2 is the default
420 since it provides additional mechanisms for confidentiality (the traffic
421 is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and
422 integrity (hmac-md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512, umac-64,
423 umac-128, hmac-ripemd160). Protocol 1 lacks a strong mechanism for
424 ensuring the integrity of the connection.
426 The methods available for authentication are: GSSAPI-based
427 authentication, host-based authentication, public key authentication,
428 challenge-response authentication, and password authentication.
429 Authentication methods are tried in the order specified above, though
430 protocol 2 has a configuration option to change the default order:
431 PreferredAuthentications.
433 Host-based authentication works as follows: If the machine the user logs
434 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote
435 machine, and the user names are the same on both sides, or if the files
436 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote
437 machine and contain a line containing the name of the client machine and
438 the name of the user on that machine, the user is considered for login.
439 Additionally, the server must be able to verify the client's host key
440 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts,
441 below) for login to be permitted. This authentication method closes
442 security holes due to IP spoofing, DNS spoofing, and routing spoofing.
443 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the
444 rlogin/rsh protocol in general, are inherently insecure and should be
445 disabled if security is desired.]
447 Public key authentication works as follows: The scheme is based on
448 public-key cryptography, using cryptosystems where encryption and
449 decryption are done using separate keys, and it is unfeasible to derive
450 the decryption key from the encryption key. The idea is that each user
451 creates a public/private key pair for authentication purposes. The
452 server knows the public key, and only the user knows the private key.
453 ssh implements public key authentication protocol automatically, using
454 one of the DSA, ECDSA, Ed25519 or RSA algorithms. Protocol 1 is
455 restricted to using only RSA keys, but protocol 2 may use any. The
456 HISTORY section of ssl(8) contains a brief discussion of the DSA and RSA
459 The file ~/.ssh/authorized_keys lists the public keys that are permitted
460 for logging in. When the user logs in, the ssh program tells the server
461 which key pair it would like to use for authentication. The client
462 proves that it has access to the private key and the server checks that
463 the corresponding public key is authorized to accept the account.
465 The user creates his/her key pair by running ssh-keygen(1). This stores
466 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol
467 2 DSA), ~/.ssh/id_ecdsa (protocol 2 ECDSA), ~/.ssh/id_ed25519 (protocol 2
468 Ed25519), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in
469 ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA),
470 ~/.ssh/id_ecdsa.pub (protocol 2 ECDSA), ~/.ssh/id_ed25519.pub (protocol 2
471 Ed25519), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home
472 directory. The user should then copy the public key to
473 ~/.ssh/authorized_keys in his/her home directory on the remote machine.
474 The authorized_keys file corresponds to the conventional ~/.rhosts file,
475 and has one key per line, though the lines can be very long. After this,
476 the user can log in without giving the password.
478 A variation on public key authentication is available in the form of
479 certificate authentication: instead of a set of public/private keys,
480 signed certificates are used. This has the advantage that a single
481 trusted certification authority can be used in place of many
482 public/private keys. See the CERTIFICATES section of ssh-keygen(1) for
485 The most convenient way to use public key or certificate authentication
486 may be with an authentication agent. See ssh-agent(1) for more
489 Challenge-response authentication works as follows: The server sends an
490 arbitrary "challenge" text, and prompts for a response. Protocol 2
491 allows multiple challenges and responses; protocol 1 is restricted to
492 just one challenge/response. Examples of challenge-response
493 authentication include BSD Authentication (see login.conf(5)) and PAM
494 (some non-OpenBSD systems).
496 Finally, if other authentication methods fail, ssh prompts the user for a
497 password. The password is sent to the remote host for checking; however,
498 since all communications are encrypted, the password cannot be seen by
499 someone listening on the network.
501 ssh automatically maintains and checks a database containing
502 identification for all hosts it has ever been used with. Host keys are
503 stored in ~/.ssh/known_hosts in the user's home directory. Additionally,
504 the file /etc/ssh/ssh_known_hosts is automatically checked for known
505 hosts. Any new hosts are automatically added to the user's file. If a
506 host's identification ever changes, ssh warns about this and disables
507 password authentication to prevent server spoofing or man-in-the-middle
508 attacks, which could otherwise be used to circumvent the encryption. The
509 StrictHostKeyChecking option can be used to control logins to machines
510 whose host key is not known or has changed.
512 When the user's identity has been accepted by the server, the server
513 either executes the given command, or logs into the machine and gives the
514 user a normal shell on the remote machine. All communication with the
515 remote command or shell will be automatically encrypted.
517 If a pseudo-terminal has been allocated (normal login session), the user
518 may use the escape characters noted below.
520 If no pseudo-tty has been allocated, the session is transparent and can
521 be used to reliably transfer binary data. On most systems, setting the
522 escape character to M-bM-^@M-^\noneM-bM-^@M-^] will also make the session transparent even if
525 The session terminates when the command or shell on the remote machine
526 exits and all X11 and TCP connections have been closed.
529 When a pseudo-terminal has been requested, ssh supports a number of
530 functions through the use of an escape character.
532 A single tilde character can be sent as ~~ or by following the tilde by a
533 character other than those described below. The escape character must
534 always follow a newline to be interpreted as special. The escape
535 character can be changed in configuration files using the EscapeChar
536 configuration directive or on the command line by the -e option.
538 The supported escapes (assuming the default M-bM-^@M-^X~M-bM-^@M-^Y) are:
544 ~# List forwarded connections.
546 ~& Background ssh at logout when waiting for forwarded connection /
547 X11 sessions to terminate.
549 ~? Display a list of escape characters.
551 ~B Send a BREAK to the remote system (only useful for SSH protocol
552 version 2 and if the peer supports it).
554 ~C Open command line. Currently this allows the addition of port
555 forwardings using the -L, -R and -D options (see above). It also
556 allows the cancellation of existing port-forwardings with
557 -KL[bind_address:]port for local, -KR[bind_address:]port for
558 remote and -KD[bind_address:]port for dynamic port-forwardings.
559 !command allows the user to execute a local command if the
560 PermitLocalCommand option is enabled in ssh_config(5). Basic
561 help is available, using the -h option.
563 ~R Request rekeying of the connection (only useful for SSH protocol
564 version 2 and if the peer supports it).
566 ~V Decrease the verbosity (LogLevel) when errors are being written
569 ~v Increase the verbosity (LogLevel) when errors are being written
573 Forwarding of arbitrary TCP connections over the secure channel can be
574 specified either on the command line or in a configuration file. One
575 possible application of TCP forwarding is a secure connection to a mail
576 server; another is going through firewalls.
578 In the example below, we look at encrypting communication between an IRC
579 client and server, even though the IRC server does not directly support
580 encrypted communications. This works as follows: the user connects to
581 the remote host using ssh, specifying a port to be used to forward
582 connections to the remote server. After that it is possible to start the
583 service which is to be encrypted on the client machine, connecting to the
584 same local port, and ssh will encrypt and forward the connection.
586 The following example tunnels an IRC session from client machine
587 M-bM-^@M-^\127.0.0.1M-bM-^@M-^] (localhost) to remote server M-bM-^@M-^\server.example.comM-bM-^@M-^]:
589 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
590 $ irc -c '#users' -p 1234 pinky 127.0.0.1
592 This tunnels a connection to IRC server M-bM-^@M-^\server.example.comM-bM-^@M-^], joining
593 channel M-bM-^@M-^\#usersM-bM-^@M-^], nickname M-bM-^@M-^\pinkyM-bM-^@M-^], using port 1234. It doesn't matter
594 which port is used, as long as it's greater than 1023 (remember, only
595 root can open sockets on privileged ports) and doesn't conflict with any
596 ports already in use. The connection is forwarded to port 6667 on the
597 remote server, since that's the standard port for IRC services.
599 The -f option backgrounds ssh and the remote command M-bM-^@M-^\sleep 10M-bM-^@M-^] is
600 specified to allow an amount of time (10 seconds, in the example) to
601 start the service which is to be tunnelled. If no connections are made
602 within the time specified, ssh will exit.
605 If the ForwardX11 variable is set to M-bM-^@M-^\yesM-bM-^@M-^] (or see the description of the
606 -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
607 environment variable is set), the connection to the X11 display is
608 automatically forwarded to the remote side in such a way that any X11
609 programs started from the shell (or command) will go through the
610 encrypted channel, and the connection to the real X server will be made
611 from the local machine. The user should not manually set DISPLAY.
612 Forwarding of X11 connections can be configured on the command line or in
615 The DISPLAY value set by ssh will point to the server machine, but with a
616 display number greater than zero. This is normal, and happens because
617 ssh creates a M-bM-^@M-^\proxyM-bM-^@M-^] X server on the server machine for forwarding the
618 connections over the encrypted channel.
620 ssh will also automatically set up Xauthority data on the server machine.
621 For this purpose, it will generate a random authorization cookie, store
622 it in Xauthority on the server, and verify that any forwarded connections
623 carry this cookie and replace it by the real cookie when the connection
624 is opened. The real authentication cookie is never sent to the server
625 machine (and no cookies are sent in the plain).
627 If the ForwardAgent variable is set to M-bM-^@M-^\yesM-bM-^@M-^] (or see the description of
628 the -A and -a options above) and the user is using an authentication
629 agent, the connection to the agent is automatically forwarded to the
633 When connecting to a server for the first time, a fingerprint of the
634 server's public key is presented to the user (unless the option
635 StrictHostKeyChecking has been disabled). Fingerprints can be determined
638 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
640 If the fingerprint is already known, it can be matched and the key can be
641 accepted or rejected. Because of the difficulty of comparing host keys
642 just by looking at fingerprint strings, there is also support to compare
643 host keys visually, using random art. By setting the VisualHostKey
644 option to M-bM-^@M-^\yesM-bM-^@M-^], a small ASCII graphic gets displayed on every login to a
645 server, no matter if the session itself is interactive or not. By
646 learning the pattern a known server produces, a user can easily find out
647 that the host key has changed when a completely different pattern is
648 displayed. Because these patterns are not unambiguous however, a pattern
649 that looks similar to the pattern remembered only gives a good
650 probability that the host key is the same, not guaranteed proof.
652 To get a listing of the fingerprints along with their random art for all
653 known hosts, the following command line can be used:
655 $ ssh-keygen -lv -f ~/.ssh/known_hosts
657 If the fingerprint is unknown, an alternative method of verification is
658 available: SSH fingerprints verified by DNS. An additional resource
659 record (RR), SSHFP, is added to a zonefile and the connecting client is
660 able to match the fingerprint with that of the key presented.
662 In this example, we are connecting a client to a server,
663 M-bM-^@M-^\host.example.comM-bM-^@M-^]. The SSHFP resource records should first be added to
664 the zonefile for host.example.com:
666 $ ssh-keygen -r host.example.com.
668 The output lines will have to be added to the zonefile. To check that
669 the zone is answering fingerprint queries:
671 $ dig -t SSHFP host.example.com
673 Finally the client connects:
675 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
677 Matching host key fingerprint found in DNS.
678 Are you sure you want to continue connecting (yes/no)?
680 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
682 SSH-BASED VIRTUAL PRIVATE NETWORKS
683 ssh contains support for Virtual Private Network (VPN) tunnelling using
684 the tun(4) network pseudo-device, allowing two networks to be joined
685 securely. The sshd_config(5) configuration option PermitTunnel controls
686 whether the server supports this, and at what level (layer 2 or 3
689 The following example would connect client network 10.0.50.0/24 with
690 remote network 10.0.99.0/24 using a point-to-point connection from
691 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway
692 to the remote network, at 192.168.1.15, allows it.
696 # ssh -f -w 0:1 192.168.1.15 true
697 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
698 # route add 10.0.99.0/24 10.1.1.2
702 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
703 # route add 10.0.50.0/24 10.1.1.1
705 Client access may be more finely tuned via the /root/.ssh/authorized_keys
706 file (see below) and the PermitRootLogin server option. The following
707 entry would permit connections on tun(4) device 1 from user M-bM-^@M-^\janeM-bM-^@M-^] and on
708 tun device 2 from user M-bM-^@M-^\johnM-bM-^@M-^], if PermitRootLogin is set to
709 M-bM-^@M-^\forced-commands-onlyM-bM-^@M-^]:
711 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
712 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
714 Since an SSH-based setup entails a fair amount of overhead, it may be
715 more suited to temporary setups, such as for wireless VPNs. More
716 permanent VPNs are better provided by tools such as ipsecctl(8) and
720 ssh will normally set the following environment variables:
722 DISPLAY The DISPLAY variable indicates the location of the
723 X11 server. It is automatically set by ssh to
724 point to a value of the form M-bM-^@M-^\hostname:nM-bM-^@M-^], where
725 M-bM-^@M-^\hostnameM-bM-^@M-^] indicates the host where the shell runs,
726 and M-bM-^@M-^XnM-bM-^@M-^Y is an integer M-bM-^IM-% 1. ssh uses this special
727 value to forward X11 connections over the secure
728 channel. The user should normally not set DISPLAY
729 explicitly, as that will render the X11 connection
730 insecure (and will require the user to manually
731 copy any required authorization cookies).
733 HOME Set to the path of the user's home directory.
735 LOGNAME Synonym for USER; set for compatibility with
736 systems that use this variable.
738 MAIL Set to the path of the user's mailbox.
740 PATH Set to the default PATH, as specified when
743 SSH_ASKPASS If ssh needs a passphrase, it will read the
744 passphrase from the current terminal if it was run
745 from a terminal. If ssh does not have a terminal
746 associated with it but DISPLAY and SSH_ASKPASS are
747 set, it will execute the program specified by
748 SSH_ASKPASS and open an X11 window to read the
749 passphrase. This is particularly useful when
750 calling ssh from a .xsession or related script.
751 (Note that on some machines it may be necessary to
752 redirect the input from /dev/null to make this
755 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
756 communicate with the agent.
758 SSH_CONNECTION Identifies the client and server ends of the
759 connection. The variable contains four space-
760 separated values: client IP address, client port
761 number, server IP address, and server port number.
763 SSH_ORIGINAL_COMMAND This variable contains the original command line if
764 a forced command is executed. It can be used to
765 extract the original arguments.
767 SSH_TTY This is set to the name of the tty (path to the
768 device) associated with the current shell or
769 command. If the current session has no tty, this
772 TZ This variable is set to indicate the present time
773 zone if it was set when the daemon was started
774 (i.e. the daemon passes the value on to new
777 USER Set to the name of the user logging in.
779 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
780 M-bM-^@M-^\VARNAME=valueM-bM-^@M-^] to the environment if the file exists and users are
781 allowed to change their environment. For more information, see the
782 PermitUserEnvironment option in sshd_config(5).
786 This file is used for host-based authentication (see above). On
787 some machines this file may need to be world-readable if the
788 user's home directory is on an NFS partition, because sshd(8)
789 reads it as root. Additionally, this file must be owned by the
790 user, and must not have write permissions for anyone else. The
791 recommended permission for most machines is read/write for the
792 user, and not accessible by others.
795 This file is used in exactly the same way as .rhosts, but allows
796 host-based authentication without permitting login with
800 This directory is the default location for all user-specific
801 configuration and authentication information. There is no
802 general requirement to keep the entire contents of this directory
803 secret, but the recommended permissions are read/write/execute
804 for the user, and not accessible by others.
806 ~/.ssh/authorized_keys
807 Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
808 for logging in as this user. The format of this file is
809 described in the sshd(8) manual page. This file is not highly
810 sensitive, but the recommended permissions are read/write for the
811 user, and not accessible by others.
814 This is the per-user configuration file. The file format and
815 configuration options are described in ssh_config(5). Because of
816 the potential for abuse, this file must have strict permissions:
817 read/write for the user, and not writable by others.
820 Contains additional definitions for environment variables; see
828 Contains the private key for authentication. These files contain
829 sensitive data and should be readable by the user but not
830 accessible by others (read/write/execute). ssh will simply
831 ignore a private key file if it is accessible by others. It is
832 possible to specify a passphrase when generating the key which
833 will be used to encrypt the sensitive part of this file using
839 ~/.ssh/id_ed25519.pub
841 Contains the public key for authentication. These files are not
842 sensitive and can (but need not) be readable by anyone.
845 Contains a list of host keys for all hosts the user has logged
846 into that are not already in the systemwide list of known host
847 keys. See sshd(8) for further details of the format of this
851 Commands in this file are executed by ssh when the user logs in,
852 just before the user's shell (or command) is started. See the
853 sshd(8) manual page for more information.
856 This file is for host-based authentication (see above). It
857 should only be writable by root.
860 This file is used in exactly the same way as hosts.equiv, but
861 allows host-based authentication without permitting login with
865 Systemwide configuration file. The file format and configuration
866 options are described in ssh_config(5).
868 /etc/ssh/ssh_host_key
869 /etc/ssh/ssh_host_dsa_key
870 /etc/ssh/ssh_host_ecdsa_key
871 /etc/ssh/ssh_host_ed25519_key
872 /etc/ssh/ssh_host_rsa_key
873 These files contain the private parts of the host keys and are
874 used for host-based authentication. If protocol version 1 is
875 used, ssh must be setuid root, since the host key is readable
876 only by root. For protocol version 2, ssh uses ssh-keysign(8) to
877 access the host keys, eliminating the requirement that ssh be
878 setuid root when host-based authentication is used. By default
879 ssh is not setuid root.
881 /etc/ssh/ssh_known_hosts
882 Systemwide list of known host keys. This file should be prepared
883 by the system administrator to contain the public host keys of
884 all machines in the organization. It should be world-readable.
885 See sshd(8) for further details of the format of this file.
888 Commands in this file are executed by ssh when the user logs in,
889 just before the user's shell (or command) is started. See the
890 sshd(8) manual page for more information.
893 ssh exits with the exit status of the remote command or with 255 if an
897 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
898 tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
901 S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
902 Numbers, RFC 4250, January 2006.
904 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture,
905 RFC 4251, January 2006.
907 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
908 RFC 4252, January 2006.
910 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer
911 Protocol, RFC 4253, January 2006.
913 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
916 J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
917 (SSH) Key Fingerprints, RFC 4255, January 2006.
919 F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
920 Secure Shell Protocol (SSH), RFC 4256, January 2006.
922 J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
923 Extension, RFC 4335, January 2006.
925 M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
926 Layer Encryption Modes, RFC 4344, January 2006.
928 B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
929 Layer Protocol, RFC 4345, January 2006.
931 M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
932 the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
934 J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File
935 Format, RFC 4716, November 2006.
937 D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the
938 Secure Shell Transport Layer, RFC 5656, December 2009.
940 A. Perrig and D. Song, Hash Visualization: a New Technique to improve
941 Real-World Security, 1999, International Workshop on Cryptographic
942 Techniques and E-Commerce (CrypTEC '99).
945 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
946 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
947 de Raadt and Dug Song removed many bugs, re-added newer features and
948 created OpenSSH. Markus Friedl contributed the support for SSH protocol
949 versions 1.5 and 2.0.
951 OpenBSD 5.7 March 3, 2015 OpenBSD 5.7
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