1 SSH(1) General Commands Manual SSH(1)
4 ssh M-bM-^@M-^S OpenSSH SSH client (remote login program)
7 ssh [-46AaCfGgKkMNnqsTtVvXxYy] [-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 [-J [user@]host[:port]] [-L address] [-l login_name] [-m mac_spec]
11 [-O ctl_cmd] [-o option] [-p port] [-Q query_option] [-R address]
12 [-S ctl_path] [-W host:port] [-w local_tun[:remote_tun]]
13 [user@]hostname [command]
16 ssh (SSH client) is a program for logging into a remote machine and for
17 executing commands on a remote machine. It is intended to provide secure
18 encrypted communications between two untrusted hosts over an insecure
19 network. X11 connections, arbitrary TCP ports and UNIX-domain sockets
20 can also be forwarded over the secure channel.
22 ssh connects and logs into the specified hostname (with optional user
23 name). The user must prove his/her identity to the remote machine using
24 one of several methods (see below).
26 If command is specified, it is executed on the remote host instead of a
29 The options are as follows:
31 -4 Forces ssh to use IPv4 addresses only.
33 -6 Forces ssh to use IPv6 addresses only.
35 -A Enables forwarding of the authentication agent connection. This
36 can also be specified on a per-host basis in a configuration
39 Agent forwarding should be enabled with caution. Users with the
40 ability to bypass file permissions on the remote host (for the
41 agent's UNIX-domain socket) can access the local agent through
42 the forwarded connection. An attacker cannot obtain key material
43 from the agent, however they can perform operations on the keys
44 that enable them to authenticate using the identities loaded into
47 -a Disables forwarding of the authentication agent connection.
50 Use bind_address on the local machine as the source address of
51 the connection. Only useful on systems with more than one
54 -C Requests compression of all data (including stdin, stdout,
55 stderr, and data for forwarded X11, TCP and UNIX-domain
56 connections). The compression algorithm is the same used by
57 gzip(1). Compression is desirable on modem lines and other slow
58 connections, but will only slow down things on fast networks.
59 The default value can be set on a host-by-host basis in the
60 configuration files; see the Compression option.
63 Selects the cipher specification for encrypting the session.
64 cipher_spec is a comma-separated list of ciphers listed in order
65 of preference. See the Ciphers keyword in ssh_config(5) for more
68 -D [bind_address:]port
69 Specifies a local M-bM-^@M-^\dynamicM-bM-^@M-^] application-level port forwarding.
70 This works by allocating a socket to listen to port on the local
71 side, optionally bound to the specified bind_address. Whenever a
72 connection is made to this port, the connection is forwarded over
73 the secure channel, and the application protocol is then used to
74 determine where to connect to from the remote machine. Currently
75 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
76 as a SOCKS server. Only root can forward privileged ports.
77 Dynamic port forwardings can also be specified in the
80 IPv6 addresses can be specified by enclosing the address in
81 square brackets. Only the superuser can forward privileged
82 ports. By default, the local port is bound in accordance with
83 the GatewayPorts setting. However, an explicit bind_address may
84 be used to bind the connection to a specific address. The
85 bind_address of M-bM-^@M-^\localhostM-bM-^@M-^] indicates that the listening port be
86 bound for local use only, while an empty address or M-bM-^@M-^X*M-bM-^@M-^Y indicates
87 that the port should be available from all interfaces.
90 Append debug logs to log_file instead of standard error.
93 Sets the escape character for sessions with a pty (default: M-bM-^@M-^X~M-bM-^@M-^Y).
94 The escape character is only recognized at the beginning of a
95 line. The escape character followed by a dot (M-bM-^@M-^X.M-bM-^@M-^Y) closes the
96 connection; followed by control-Z suspends the connection; and
97 followed by itself sends the escape character once. Setting the
98 character to M-bM-^@M-^\noneM-bM-^@M-^] disables any escapes and makes the session
102 Specifies an alternative per-user configuration file. If a
103 configuration file is given on the command line, the system-wide
104 configuration file (/etc/ssh/ssh_config) will be ignored. The
105 default for the per-user configuration file is ~/.ssh/config.
107 -f Requests ssh to go to background just before command execution.
108 This is useful if ssh is going to ask for passwords or
109 passphrases, but the user wants it in the background. This
110 implies -n. The recommended way to start X11 programs at a
111 remote site is with something like ssh -f host xterm.
113 If the ExitOnForwardFailure configuration option is set to M-bM-^@M-^\yesM-bM-^@M-^],
114 then a client started with -f will wait for all remote port
115 forwards to be successfully established before placing itself in
118 -G Causes ssh to print its configuration after evaluating Host and
119 Match blocks and exit.
121 -g Allows remote hosts to connect to local forwarded ports. If used
122 on a multiplexed connection, then this option must be specified
123 on the master process.
126 Specify the PKCS#11 shared library ssh should use to communicate
127 with a PKCS#11 token providing the user's private RSA key.
130 Selects a file from which the identity (private key) for public
131 key authentication is read. The default is ~/.ssh/id_dsa,
132 ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 and ~/.ssh/id_rsa. Identity
133 files may also be specified on a per-host basis in the
134 configuration file. It is possible to have multiple -i options
135 (and multiple identities specified in configuration files). If
136 no certificates have been explicitly specified by the
137 CertificateFile directive, ssh will also try to load certificate
138 information from the filename obtained by appending -cert.pub to
141 -J [user@]host[:port]
142 Connect to the target host by first making a ssh connection to
143 the jump host and then establishing a TCP forwarding to the
144 ultimate destination from there. Multiple jump hops may be
145 specified separated by comma characters. This is a shortcut to
146 specify a ProxyJump configuration directive.
148 -K Enables GSSAPI-based authentication and forwarding (delegation)
149 of GSSAPI credentials to the server.
151 -k Disables forwarding (delegation) of GSSAPI credentials to the
154 -L [bind_address:]port:host:hostport
155 -L [bind_address:]port:remote_socket
156 -L local_socket:host:hostport
157 -L local_socket:remote_socket
158 Specifies that connections to the given TCP port or Unix socket
159 on the local (client) host are to be forwarded to the given host
160 and port, or Unix socket, on the remote side. This works by
161 allocating a socket to listen to either a TCP port on the local
162 side, optionally bound to the specified bind_address, or to a
163 Unix socket. Whenever a connection is made to the local port or
164 socket, the connection is forwarded over the secure channel, and
165 a connection is made to either host port hostport, or the Unix
166 socket remote_socket, from the remote machine.
168 Port forwardings can also be specified in the configuration file.
169 Only the superuser can forward privileged ports. IPv6 addresses
170 can be specified by enclosing the address in square brackets.
172 By default, the local port is bound in accordance with the
173 GatewayPorts setting. However, an explicit bind_address may be
174 used to bind the connection to a specific address. The
175 bind_address of M-bM-^@M-^\localhostM-bM-^@M-^] indicates that the listening port be
176 bound for local use only, while an empty address or M-bM-^@M-^X*M-bM-^@M-^Y indicates
177 that the port should be available from all interfaces.
180 Specifies the user to log in as on the remote machine. This also
181 may be specified on a per-host basis in the configuration file.
183 -M Places the ssh client into M-bM-^@M-^\masterM-bM-^@M-^] mode for connection sharing.
184 Multiple -M options places ssh into M-bM-^@M-^\masterM-bM-^@M-^] mode with
185 confirmation required before slave connections are accepted.
186 Refer to the description of ControlMaster in ssh_config(5) for
190 A comma-separated list of MAC (message authentication code)
191 algorithms, specified in order of preference. See the MACs
192 keyword for more information.
194 -N Do not execute a remote command. This is useful for just
197 -n Redirects stdin from /dev/null (actually, prevents reading from
198 stdin). This must be used when ssh is run in the background. A
199 common trick is to use this to run X11 programs on a remote
200 machine. For example, ssh -n shadows.cs.hut.fi emacs & will
201 start an emacs on shadows.cs.hut.fi, and the X11 connection will
202 be automatically forwarded over an encrypted channel. The ssh
203 program will be put in the background. (This does not work if
204 ssh needs to ask for a password or passphrase; see also the -f
208 Control an active connection multiplexing master process. When
209 the -O option is specified, the ctl_cmd argument is interpreted
210 and passed to the master process. Valid commands are: M-bM-^@M-^\checkM-bM-^@M-^]
211 (check that the master process is running), M-bM-^@M-^\forwardM-bM-^@M-^] (request
212 forwardings without command execution), M-bM-^@M-^\cancelM-bM-^@M-^] (cancel
213 forwardings), M-bM-^@M-^\exitM-bM-^@M-^] (request the master to exit), and M-bM-^@M-^\stopM-bM-^@M-^]
214 (request the master to stop accepting further multiplexing
218 Can be used to give options in the format used in the
219 configuration file. This is useful for specifying options for
220 which there is no separate command-line flag. For full details
221 of the options listed below, and their possible values, see
229 CanonicalizeFallbackLocal
232 CanonicalizePermittedCNAMEs
234 ChallengeResponseAuthentication
255 GSSAPIDelegateCredentials
258 HostbasedAuthentication
268 KbdInteractiveAuthentication
269 KbdInteractiveDevices
276 NoHostAuthenticationForLocalhost
277 NumberOfPasswordPrompts
278 PasswordAuthentication
282 PreferredAuthentications
286 PubkeyAcceptedKeyTypes
296 StreamLocalBindUnlink
297 StrictHostKeyChecking
310 Port to connect to on the remote host. This can be specified on
311 a per-host basis in the configuration file.
314 Queries ssh for the algorithms supported for the specified
315 version 2. The available features are: cipher (supported
316 symmetric ciphers), cipher-auth (supported symmetric ciphers that
317 support authenticated encryption), mac (supported message
318 integrity codes), kex (key exchange algorithms), key (key types),
319 key-cert (certificate key types), key-plain (non-certificate key
320 types), and protocol-version (supported SSH protocol versions).
322 -q Quiet mode. Causes most warning and diagnostic messages to be
325 -R [bind_address:]port:host:hostport
326 -R [bind_address:]port:local_socket
327 -R remote_socket:host:hostport
328 -R remote_socket:local_socket
329 -R [bind_address:]port
330 Specifies that connections to the given TCP port or Unix socket
331 on the remote (server) host are to be forwarded to the local
334 This works by allocating a socket to listen to either a TCP port
335 or to a Unix socket on the remote side. Whenever a connection is
336 made to this port or Unix socket, the connection is forwarded
337 over the secure channel, and a connection is made from the local
338 machine to either an explicit destination specified by host port
339 hostport, or local_socket, or, if no explicit destination was
340 specified, ssh will act as a SOCKS 4/5 proxy and forward
341 connections to the destinations requested by the remote SOCKS
344 Port forwardings can also be specified in the configuration file.
345 Privileged ports can be forwarded only when logging in as root on
346 the remote machine. IPv6 addresses can be specified by enclosing
347 the address in square brackets.
349 By default, TCP listening sockets on the server will be bound to
350 the loopback interface only. This may be overridden by
351 specifying a bind_address. An empty bind_address, or the address
352 M-bM-^@M-^X*M-bM-^@M-^Y, indicates that the remote socket should listen on all
353 interfaces. Specifying a remote bind_address will only succeed
354 if the server's GatewayPorts option is enabled (see
357 If the port argument is M-bM-^@M-^X0M-bM-^@M-^Y, the listen port will be dynamically
358 allocated on the server and reported to the client at run time.
359 When used together with -O forward the allocated port will be
360 printed to the standard output.
363 Specifies the location of a control socket for connection
364 sharing, or the string M-bM-^@M-^\noneM-bM-^@M-^] to disable connection sharing.
365 Refer to the description of ControlPath and ControlMaster in
366 ssh_config(5) for details.
368 -s May be used to request invocation of a subsystem on the remote
369 system. Subsystems facilitate the use of SSH as a secure
370 transport for other applications (e.g. sftp(1)). The subsystem
371 is specified as the remote command.
373 -T Disable pseudo-terminal allocation.
375 -t Force pseudo-terminal allocation. This can be used to execute
376 arbitrary screen-based programs on a remote machine, which can be
377 very useful, e.g. when implementing menu services. Multiple -t
378 options force tty allocation, even if ssh has no local tty.
380 -V Display the version number and exit.
382 -v Verbose mode. Causes ssh to print debugging messages about its
383 progress. This is helpful in debugging connection,
384 authentication, and configuration problems. Multiple -v options
385 increase the verbosity. The maximum is 3.
388 Requests that standard input and output on the client be
389 forwarded to host on port over the secure channel. Implies -N,
390 -T, ExitOnForwardFailure and ClearAllForwardings, though these
391 can be overridden in the configuration file or using -o command
394 -w local_tun[:remote_tun]
395 Requests tunnel device forwarding with the specified tun(4)
396 devices between the client (local_tun) and the server
399 The devices may be specified by numerical ID or the keyword
400 M-bM-^@M-^\anyM-bM-^@M-^], which uses the next available tunnel device. If
401 remote_tun is not specified, it defaults to M-bM-^@M-^\anyM-bM-^@M-^]. See also the
402 Tunnel and TunnelDevice directives in ssh_config(5). If the
403 Tunnel directive is unset, it is set to the default tunnel mode,
404 which is M-bM-^@M-^\point-to-pointM-bM-^@M-^].
406 -X Enables X11 forwarding. This can also be specified on a per-host
407 basis in a configuration file.
409 X11 forwarding should be enabled with caution. Users with the
410 ability to bypass file permissions on the remote host (for the
411 user's X authorization database) can access the local X11 display
412 through the forwarded connection. An attacker may then be able
413 to perform activities such as keystroke monitoring.
415 For this reason, X11 forwarding is subjected to X11 SECURITY
416 extension restrictions by default. Please refer to the ssh -Y
417 option and the ForwardX11Trusted directive in ssh_config(5) for
420 -x Disables X11 forwarding.
422 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
423 subjected to the X11 SECURITY extension controls.
425 -y Send log information using the syslog(3) system module. By
426 default this information is sent to stderr.
428 ssh may additionally obtain configuration data from a per-user
429 configuration file and a system-wide configuration file. The file format
430 and configuration options are described in ssh_config(5).
433 The OpenSSH SSH client supports SSH protocol 2.
435 The methods available for authentication are: GSSAPI-based
436 authentication, host-based authentication, public key authentication,
437 challenge-response authentication, and password authentication.
438 Authentication methods are tried in the order specified above, though
439 PreferredAuthentications can be used to change the default order.
441 Host-based authentication works as follows: If the machine the user logs
442 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote
443 machine, and the user names are the same on both sides, or if the files
444 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote
445 machine and contain a line containing the name of the client machine and
446 the name of the user on that machine, the user is considered for login.
447 Additionally, the server must be able to verify the client's host key
448 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts,
449 below) for login to be permitted. This authentication method closes
450 security holes due to IP spoofing, DNS spoofing, and routing spoofing.
451 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the
452 rlogin/rsh protocol in general, are inherently insecure and should be
453 disabled if security is desired.]
455 Public key authentication works as follows: The scheme is based on
456 public-key cryptography, using cryptosystems where encryption and
457 decryption are done using separate keys, and it is unfeasible to derive
458 the decryption key from the encryption key. The idea is that each user
459 creates a public/private key pair for authentication purposes. The
460 server knows the public key, and only the user knows the private key.
461 ssh implements public key authentication protocol automatically, using
462 one of the DSA, ECDSA, Ed25519 or RSA algorithms. The HISTORY section of
463 ssl(8) contains a brief discussion of the DSA and RSA algorithms.
465 The file ~/.ssh/authorized_keys lists the public keys that are permitted
466 for logging in. When the user logs in, the ssh program tells the server
467 which key pair it would like to use for authentication. The client
468 proves that it has access to the private key and the server checks that
469 the corresponding public key is authorized to accept the account.
471 The server may inform the client of errors that prevented public key
472 authentication from succeeding after authentication completes using a
473 different method. These may be viewed by increasing the LogLevel to
474 DEBUG or higher (e.g. by using the -v flag).
476 The user creates his/her key pair by running ssh-keygen(1). This stores
477 the private key in ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA),
478 ~/.ssh/id_ed25519 (Ed25519), or ~/.ssh/id_rsa (RSA) and stores the public
479 key in ~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub (ECDSA),
480 ~/.ssh/id_ed25519.pub (Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's
481 home directory. The user should then copy the public key to
482 ~/.ssh/authorized_keys in his/her home directory on the remote machine.
483 The authorized_keys file corresponds to the conventional ~/.rhosts file,
484 and has one key per line, though the lines can be very long. After this,
485 the user can log in without giving the password.
487 A variation on public key authentication is available in the form of
488 certificate authentication: instead of a set of public/private keys,
489 signed certificates are used. This has the advantage that a single
490 trusted certification authority can be used in place of many
491 public/private keys. See the CERTIFICATES section of ssh-keygen(1) for
494 The most convenient way to use public key or certificate authentication
495 may be with an authentication agent. See ssh-agent(1) and (optionally)
496 the AddKeysToAgent directive in ssh_config(5) for more information.
498 Challenge-response authentication works as follows: The server sends an
499 arbitrary "challenge" text, and prompts for a response. Examples of
500 challenge-response authentication include BSD Authentication (see
501 login.conf(5)) and PAM (some non-OpenBSD systems).
503 Finally, if other authentication methods fail, ssh prompts the user for a
504 password. The password is sent to the remote host for checking; however,
505 since all communications are encrypted, the password cannot be seen by
506 someone listening on the network.
508 ssh automatically maintains and checks a database containing
509 identification for all hosts it has ever been used with. Host keys are
510 stored in ~/.ssh/known_hosts in the user's home directory. Additionally,
511 the file /etc/ssh/ssh_known_hosts is automatically checked for known
512 hosts. Any new hosts are automatically added to the user's file. If a
513 host's identification ever changes, ssh warns about this and disables
514 password authentication to prevent server spoofing or man-in-the-middle
515 attacks, which could otherwise be used to circumvent the encryption. The
516 StrictHostKeyChecking option can be used to control logins to machines
517 whose host key is not known or has changed.
519 When the user's identity has been accepted by the server, the server
520 either executes the given command in a non-interactive session or, if no
521 command has been specified, logs into the machine and gives the user a
522 normal shell as an interactive session. All communication with the
523 remote command or shell will be automatically encrypted.
525 If an interactive session is requested ssh by default will only request a
526 pseudo-terminal (pty) for interactive sessions when the client has one.
527 The flags -T and -t can be used to override this behaviour.
529 If a pseudo-terminal has been allocated the user may use the escape
530 characters noted below.
532 If no pseudo-terminal has been allocated, the session is transparent and
533 can be used to reliably transfer binary data. On most systems, setting
534 the escape character to M-bM-^@M-^\noneM-bM-^@M-^] will also make the session transparent
535 even if a tty is used.
537 The session terminates when the command or shell on the remote machine
538 exits and all X11 and TCP connections have been closed.
541 When a pseudo-terminal has been requested, ssh supports a number of
542 functions through the use of an escape character.
544 A single tilde character can be sent as ~~ or by following the tilde by a
545 character other than those described below. The escape character must
546 always follow a newline to be interpreted as special. The escape
547 character can be changed in configuration files using the EscapeChar
548 configuration directive or on the command line by the -e option.
550 The supported escapes (assuming the default M-bM-^@M-^X~M-bM-^@M-^Y) are:
556 ~# List forwarded connections.
558 ~& Background ssh at logout when waiting for forwarded connection /
559 X11 sessions to terminate.
561 ~? Display a list of escape characters.
563 ~B Send a BREAK to the remote system (only useful if the peer
566 ~C Open command line. Currently this allows the addition of port
567 forwardings using the -L, -R and -D options (see above). It also
568 allows the cancellation of existing port-forwardings with
569 -KL[bind_address:]port for local, -KR[bind_address:]port for
570 remote and -KD[bind_address:]port for dynamic port-forwardings.
571 !command allows the user to execute a local command if the
572 PermitLocalCommand option is enabled in ssh_config(5). Basic
573 help is available, using the -h option.
575 ~R Request rekeying of the connection (only useful if the peer
578 ~V Decrease the verbosity (LogLevel) when errors are being written
581 ~v Increase the verbosity (LogLevel) when errors are being written
585 Forwarding of arbitrary TCP connections over the secure channel can be
586 specified either on the command line or in a configuration file. One
587 possible application of TCP forwarding is a secure connection to a mail
588 server; another is going through firewalls.
590 In the example below, we look at encrypting communication between an IRC
591 client and server, even though the IRC server does not directly support
592 encrypted communications. This works as follows: the user connects to
593 the remote host using ssh, specifying a port to be used to forward
594 connections to the remote server. After that it is possible to start the
595 service which is to be encrypted on the client machine, connecting to the
596 same local port, and ssh will encrypt and forward the connection.
598 The following example tunnels an IRC session from client machine
599 M-bM-^@M-^\127.0.0.1M-bM-^@M-^] (localhost) to remote server M-bM-^@M-^\server.example.comM-bM-^@M-^]:
601 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
602 $ irc -c '#users' -p 1234 pinky 127.0.0.1
604 This tunnels a connection to IRC server M-bM-^@M-^\server.example.comM-bM-^@M-^], joining
605 channel M-bM-^@M-^\#usersM-bM-^@M-^], nickname M-bM-^@M-^\pinkyM-bM-^@M-^], using port 1234. It doesn't matter
606 which port is used, as long as it's greater than 1023 (remember, only
607 root can open sockets on privileged ports) and doesn't conflict with any
608 ports already in use. The connection is forwarded to port 6667 on the
609 remote server, since that's the standard port for IRC services.
611 The -f option backgrounds ssh and the remote command M-bM-^@M-^\sleep 10M-bM-^@M-^] is
612 specified to allow an amount of time (10 seconds, in the example) to
613 start the service which is to be tunnelled. If no connections are made
614 within the time specified, ssh will exit.
617 If the ForwardX11 variable is set to M-bM-^@M-^\yesM-bM-^@M-^] (or see the description of the
618 -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
619 environment variable is set), the connection to the X11 display is
620 automatically forwarded to the remote side in such a way that any X11
621 programs started from the shell (or command) will go through the
622 encrypted channel, and the connection to the real X server will be made
623 from the local machine. The user should not manually set DISPLAY.
624 Forwarding of X11 connections can be configured on the command line or in
627 The DISPLAY value set by ssh will point to the server machine, but with a
628 display number greater than zero. This is normal, and happens because
629 ssh creates a M-bM-^@M-^\proxyM-bM-^@M-^] X server on the server machine for forwarding the
630 connections over the encrypted channel.
632 ssh will also automatically set up Xauthority data on the server machine.
633 For this purpose, it will generate a random authorization cookie, store
634 it in Xauthority on the server, and verify that any forwarded connections
635 carry this cookie and replace it by the real cookie when the connection
636 is opened. The real authentication cookie is never sent to the server
637 machine (and no cookies are sent in the plain).
639 If the ForwardAgent variable is set to M-bM-^@M-^\yesM-bM-^@M-^] (or see the description of
640 the -A and -a options above) and the user is using an authentication
641 agent, the connection to the agent is automatically forwarded to the
645 When connecting to a server for the first time, a fingerprint of the
646 server's public key is presented to the user (unless the option
647 StrictHostKeyChecking has been disabled). Fingerprints can be determined
650 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
652 If the fingerprint is already known, it can be matched and the key can be
653 accepted or rejected. If only legacy (MD5) fingerprints for the server
654 are available, the ssh-keygen(1) -E option may be used to downgrade the
655 fingerprint algorithm to match.
657 Because of the difficulty of comparing host keys just by looking at
658 fingerprint strings, there is also support to compare host keys visually,
659 using random art. By setting the VisualHostKey option to M-bM-^@M-^\yesM-bM-^@M-^], a small
660 ASCII graphic gets displayed on every login to a server, no matter if the
661 session itself is interactive or not. By learning the pattern a known
662 server produces, a user can easily find out that the host key has changed
663 when a completely different pattern is displayed. Because these patterns
664 are not unambiguous however, a pattern that looks similar to the pattern
665 remembered only gives a good probability that the host key is the same,
666 not guaranteed proof.
668 To get a listing of the fingerprints along with their random art for all
669 known hosts, the following command line can be used:
671 $ ssh-keygen -lv -f ~/.ssh/known_hosts
673 If the fingerprint is unknown, an alternative method of verification is
674 available: SSH fingerprints verified by DNS. An additional resource
675 record (RR), SSHFP, is added to a zonefile and the connecting client is
676 able to match the fingerprint with that of the key presented.
678 In this example, we are connecting a client to a server,
679 M-bM-^@M-^\host.example.comM-bM-^@M-^]. The SSHFP resource records should first be added to
680 the zonefile for host.example.com:
682 $ ssh-keygen -r host.example.com.
684 The output lines will have to be added to the zonefile. To check that
685 the zone is answering fingerprint queries:
687 $ dig -t SSHFP host.example.com
689 Finally the client connects:
691 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
693 Matching host key fingerprint found in DNS.
694 Are you sure you want to continue connecting (yes/no)?
696 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
698 SSH-BASED VIRTUAL PRIVATE NETWORKS
699 ssh contains support for Virtual Private Network (VPN) tunnelling using
700 the tun(4) network pseudo-device, allowing two networks to be joined
701 securely. The sshd_config(5) configuration option PermitTunnel controls
702 whether the server supports this, and at what level (layer 2 or 3
705 The following example would connect client network 10.0.50.0/24 with
706 remote network 10.0.99.0/24 using a point-to-point connection from
707 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway
708 to the remote network, at 192.168.1.15, allows it.
712 # ssh -f -w 0:1 192.168.1.15 true
713 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
714 # route add 10.0.99.0/24 10.1.1.2
718 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
719 # route add 10.0.50.0/24 10.1.1.1
721 Client access may be more finely tuned via the /root/.ssh/authorized_keys
722 file (see below) and the PermitRootLogin server option. The following
723 entry would permit connections on tun(4) device 1 from user M-bM-^@M-^\janeM-bM-^@M-^] and on
724 tun device 2 from user M-bM-^@M-^\johnM-bM-^@M-^], if PermitRootLogin is set to
725 M-bM-^@M-^\forced-commands-onlyM-bM-^@M-^]:
727 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
728 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
730 Since an SSH-based setup entails a fair amount of overhead, it may be
731 more suited to temporary setups, such as for wireless VPNs. More
732 permanent VPNs are better provided by tools such as ipsecctl(8) and
736 ssh will normally set the following environment variables:
738 DISPLAY The DISPLAY variable indicates the location of the
739 X11 server. It is automatically set by ssh to
740 point to a value of the form M-bM-^@M-^\hostname:nM-bM-^@M-^], where
741 M-bM-^@M-^\hostnameM-bM-^@M-^] indicates the host where the shell runs,
742 and M-bM-^@M-^XnM-bM-^@M-^Y is an integer M-bM-^IM-% 1. ssh uses this special
743 value to forward X11 connections over the secure
744 channel. The user should normally not set DISPLAY
745 explicitly, as that will render the X11 connection
746 insecure (and will require the user to manually
747 copy any required authorization cookies).
749 HOME Set to the path of the user's home directory.
751 LOGNAME Synonym for USER; set for compatibility with
752 systems that use this variable.
754 MAIL Set to the path of the user's mailbox.
756 PATH Set to the default PATH, as specified when
759 SSH_ASKPASS If ssh needs a passphrase, it will read the
760 passphrase from the current terminal if it was run
761 from a terminal. If ssh does not have a terminal
762 associated with it but DISPLAY and SSH_ASKPASS are
763 set, it will execute the program specified by
764 SSH_ASKPASS and open an X11 window to read the
765 passphrase. This is particularly useful when
766 calling ssh from a .xsession or related script.
767 (Note that on some machines it may be necessary to
768 redirect the input from /dev/null to make this
771 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
772 communicate with the agent.
774 SSH_CONNECTION Identifies the client and server ends of the
775 connection. The variable contains four space-
776 separated values: client IP address, client port
777 number, server IP address, and server port number.
779 SSH_ORIGINAL_COMMAND This variable contains the original command line if
780 a forced command is executed. It can be used to
781 extract the original arguments.
783 SSH_TTY This is set to the name of the tty (path to the
784 device) associated with the current shell or
785 command. If the current session has no tty, this
788 TZ This variable is set to indicate the present time
789 zone if it was set when the daemon was started
790 (i.e. the daemon passes the value on to new
793 USER Set to the name of the user logging in.
795 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
796 M-bM-^@M-^\VARNAME=valueM-bM-^@M-^] to the environment if the file exists and users are
797 allowed to change their environment. For more information, see the
798 PermitUserEnvironment option in sshd_config(5).
802 This file is used for host-based authentication (see above). On
803 some machines this file may need to be world-readable if the
804 user's home directory is on an NFS partition, because sshd(8)
805 reads it as root. Additionally, this file must be owned by the
806 user, and must not have write permissions for anyone else. The
807 recommended permission for most machines is read/write for the
808 user, and not accessible by others.
811 This file is used in exactly the same way as .rhosts, but allows
812 host-based authentication without permitting login with
816 This directory is the default location for all user-specific
817 configuration and authentication information. There is no
818 general requirement to keep the entire contents of this directory
819 secret, but the recommended permissions are read/write/execute
820 for the user, and not accessible by others.
822 ~/.ssh/authorized_keys
823 Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
824 for logging in as this user. The format of this file is
825 described in the sshd(8) manual page. This file is not highly
826 sensitive, but the recommended permissions are read/write for the
827 user, and not accessible by others.
830 This is the per-user configuration file. The file format and
831 configuration options are described in ssh_config(5). Because of
832 the potential for abuse, this file must have strict permissions:
833 read/write for the user, and not writable by others.
836 Contains additional definitions for environment variables; see
843 Contains the private key for authentication. These files contain
844 sensitive data and should be readable by the user but not
845 accessible by others (read/write/execute). ssh will simply
846 ignore a private key file if it is accessible by others. It is
847 possible to specify a passphrase when generating the key which
848 will be used to encrypt the sensitive part of this file using
853 ~/.ssh/id_ed25519.pub
855 Contains the public key for authentication. These files are not
856 sensitive and can (but need not) be readable by anyone.
859 Contains a list of host keys for all hosts the user has logged
860 into that are not already in the systemwide list of known host
861 keys. See sshd(8) for further details of the format of this
865 Commands in this file are executed by ssh when the user logs in,
866 just before the user's shell (or command) is started. See the
867 sshd(8) manual page for more information.
870 This file is for host-based authentication (see above). It
871 should only be writable by root.
874 This file is used in exactly the same way as hosts.equiv, but
875 allows host-based authentication without permitting login with
879 Systemwide configuration file. The file format and configuration
880 options are described in ssh_config(5).
882 /etc/ssh/ssh_host_key
883 /etc/ssh/ssh_host_dsa_key
884 /etc/ssh/ssh_host_ecdsa_key
885 /etc/ssh/ssh_host_ed25519_key
886 /etc/ssh/ssh_host_rsa_key
887 These files contain the private parts of the host keys and are
888 used for host-based authentication.
890 /etc/ssh/ssh_known_hosts
891 Systemwide list of known host keys. This file should be prepared
892 by the system administrator to contain the public host keys of
893 all machines in the organization. It should be world-readable.
894 See sshd(8) for further details of the format of this file.
897 Commands in this file are executed by ssh when the user logs in,
898 just before the user's shell (or command) is started. See the
899 sshd(8) manual page for more information.
902 ssh exits with the exit status of the remote command or with 255 if an
906 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
907 tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
910 S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
911 Numbers, RFC 4250, January 2006.
913 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture,
914 RFC 4251, January 2006.
916 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
917 RFC 4252, January 2006.
919 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer
920 Protocol, RFC 4253, January 2006.
922 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
925 J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
926 (SSH) Key Fingerprints, RFC 4255, January 2006.
928 F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
929 Secure Shell Protocol (SSH), RFC 4256, January 2006.
931 J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
932 Extension, RFC 4335, January 2006.
934 M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
935 Layer Encryption Modes, RFC 4344, January 2006.
937 B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
938 Layer Protocol, RFC 4345, January 2006.
940 M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
941 the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
943 J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File
944 Format, RFC 4716, November 2006.
946 D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the
947 Secure Shell Transport Layer, RFC 5656, December 2009.
949 A. Perrig and D. Song, Hash Visualization: a New Technique to improve
950 Real-World Security, 1999, International Workshop on Cryptographic
951 Techniques and E-Commerce (CrypTEC '99).
954 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
955 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
956 de Raadt and Dug Song removed many bugs, re-added newer features and
957 created OpenSSH. Markus Friedl contributed the support for SSH protocol
958 versions 1.5 and 2.0.
960 OpenBSD 6.2 September 21, 2017 OpenBSD 6.2
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