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_interface] [-b bind_address]
8 [-c cipher_spec] [-D [bind_address:]port] [-E log_file]
9 [-e escape_char] [-F configfile] [-I pkcs11] [-i identity_file]
10 [-J destination] [-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]] destination
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 destination, which may be
23 specified as either [user@]hostname or a URI of the form
24 ssh://[user@]hostname[:port]. The user must prove his/her identity to
25 the remote machine using one of several methods (see below).
27 If a command is specified, it is executed on the remote host instead of a
30 The options are as follows:
32 -4 Forces ssh to use IPv4 addresses only.
34 -6 Forces ssh to use IPv6 addresses only.
36 -A Enables forwarding of the authentication agent connection. This
37 can also be specified on a per-host basis in a configuration
40 Agent forwarding should be enabled with caution. Users with the
41 ability to bypass file permissions on the remote host (for the
42 agent's UNIX-domain socket) can access the local agent through
43 the forwarded connection. An attacker cannot obtain key material
44 from the agent, however they can perform operations on the keys
45 that enable them to authenticate using the identities loaded into
48 -a Disables forwarding of the authentication agent connection.
51 Bind to the address of bind_interface before attempting to
52 connect to the destination host. This is only useful on systems
53 with more than one address.
56 Use bind_address on the local machine as the source address of
57 the connection. Only useful on systems with more than one
60 -C Requests compression of all data (including stdin, stdout,
61 stderr, and data for forwarded X11, TCP and UNIX-domain
62 connections). The compression algorithm is the same used by
63 gzip(1). Compression is desirable on modem lines and other slow
64 connections, but will only slow down things on fast networks.
65 The default value can be set on a host-by-host basis in the
66 configuration files; see the Compression option.
69 Selects the cipher specification for encrypting the session.
70 cipher_spec is a comma-separated list of ciphers listed in order
71 of preference. See the Ciphers keyword in ssh_config(5) for more
74 -D [bind_address:]port
75 Specifies a local M-bM-^@M-^\dynamicM-bM-^@M-^] application-level port forwarding.
76 This works by allocating a socket to listen to port on the local
77 side, optionally bound to the specified bind_address. Whenever a
78 connection is made to this port, the connection is forwarded over
79 the secure channel, and the application protocol is then used to
80 determine where to connect to from the remote machine. Currently
81 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
82 as a SOCKS server. Only root can forward privileged ports.
83 Dynamic port forwardings can also be specified in the
86 IPv6 addresses can be specified by enclosing the address in
87 square brackets. Only the superuser can forward privileged
88 ports. By default, the local port is bound in accordance with
89 the GatewayPorts setting. However, an explicit bind_address may
90 be used to bind the connection to a specific address. The
91 bind_address of M-bM-^@M-^\localhostM-bM-^@M-^] indicates that the listening port be
92 bound for local use only, while an empty address or M-bM-^@M-^X*M-bM-^@M-^Y indicates
93 that the port should be available from all interfaces.
96 Append debug logs to log_file instead of standard error.
99 Sets the escape character for sessions with a pty (default: M-bM-^@M-^X~M-bM-^@M-^Y).
100 The escape character is only recognized at the beginning of a
101 line. The escape character followed by a dot (M-bM-^@M-^X.M-bM-^@M-^Y) closes the
102 connection; followed by control-Z suspends the connection; and
103 followed by itself sends the escape character once. Setting the
104 character to M-bM-^@M-^\noneM-bM-^@M-^] disables any escapes and makes the session
108 Specifies an alternative per-user configuration file. If a
109 configuration file is given on the command line, the system-wide
110 configuration file (/etc/ssh/ssh_config) will be ignored. The
111 default for the per-user configuration file is ~/.ssh/config.
113 -f Requests ssh to go to background just before command execution.
114 This is useful if ssh is going to ask for passwords or
115 passphrases, but the user wants it in the background. This
116 implies -n. The recommended way to start X11 programs at a
117 remote site is with something like ssh -f host xterm.
119 If the ExitOnForwardFailure configuration option is set to M-bM-^@M-^\yesM-bM-^@M-^],
120 then a client started with -f will wait for all remote port
121 forwards to be successfully established before placing itself in
124 -G Causes ssh to print its configuration after evaluating Host and
125 Match blocks and exit.
127 -g Allows remote hosts to connect to local forwarded ports. If used
128 on a multiplexed connection, then this option must be specified
129 on the master process.
132 Specify the PKCS#11 shared library ssh should use to communicate
133 with a PKCS#11 token providing the user's private RSA key.
136 Selects a file from which the identity (private key) for public
137 key authentication is read. The default is ~/.ssh/id_dsa,
138 ~/.ssh/id_ecdsa, ~/.ssh/id_ed25519 and ~/.ssh/id_rsa. Identity
139 files may also be specified on a per-host basis in the
140 configuration file. It is possible to have multiple -i options
141 (and multiple identities specified in configuration files). If
142 no certificates have been explicitly specified by the
143 CertificateFile directive, ssh will also try to load certificate
144 information from the filename obtained by appending -cert.pub to
148 Connect to the target host by first making a ssh connection to
149 the jump host described by destination and then establishing a
150 TCP forwarding to the ultimate destination from there. Multiple
151 jump hops may be specified separated by comma characters. This
152 is a shortcut to specify a ProxyJump configuration directive.
154 -K Enables GSSAPI-based authentication and forwarding (delegation)
155 of GSSAPI credentials to the server.
157 -k Disables forwarding (delegation) of GSSAPI credentials to the
160 -L [bind_address:]port:host:hostport
161 -L [bind_address:]port:remote_socket
162 -L local_socket:host:hostport
163 -L local_socket:remote_socket
164 Specifies that connections to the given TCP port or Unix socket
165 on the local (client) host are to be forwarded to the given host
166 and port, or Unix socket, on the remote side. This works by
167 allocating a socket to listen to either a TCP port on the local
168 side, optionally bound to the specified bind_address, or to a
169 Unix socket. Whenever a connection is made to the local port or
170 socket, the connection is forwarded over the secure channel, and
171 a connection is made to either host port hostport, or the Unix
172 socket remote_socket, from the remote machine.
174 Port forwardings can also be specified in the configuration file.
175 Only the superuser can forward privileged ports. IPv6 addresses
176 can be specified by enclosing the address in square brackets.
178 By default, the local port is bound in accordance with the
179 GatewayPorts setting. However, an explicit bind_address may be
180 used to bind the connection to a specific address. The
181 bind_address of M-bM-^@M-^\localhostM-bM-^@M-^] indicates that the listening port be
182 bound for local use only, while an empty address or M-bM-^@M-^X*M-bM-^@M-^Y indicates
183 that the port should be available from all interfaces.
186 Specifies the user to log in as on the remote machine. This also
187 may be specified on a per-host basis in the configuration file.
189 -M Places the ssh client into M-bM-^@M-^\masterM-bM-^@M-^] mode for connection sharing.
190 Multiple -M options places ssh into M-bM-^@M-^\masterM-bM-^@M-^] mode with
191 confirmation required before slave connections are accepted.
192 Refer to the description of ControlMaster in ssh_config(5) for
196 A comma-separated list of MAC (message authentication code)
197 algorithms, specified in order of preference. See the MACs
198 keyword for more information.
200 -N Do not execute a remote command. This is useful for just
203 -n Redirects stdin from /dev/null (actually, prevents reading from
204 stdin). This must be used when ssh is run in the background. A
205 common trick is to use this to run X11 programs on a remote
206 machine. For example, ssh -n shadows.cs.hut.fi emacs & will
207 start an emacs on shadows.cs.hut.fi, and the X11 connection will
208 be automatically forwarded over an encrypted channel. The ssh
209 program will be put in the background. (This does not work if
210 ssh needs to ask for a password or passphrase; see also the -f
214 Control an active connection multiplexing master process. When
215 the -O option is specified, the ctl_cmd argument is interpreted
216 and passed to the master process. Valid commands are: M-bM-^@M-^\checkM-bM-^@M-^]
217 (check that the master process is running), M-bM-^@M-^\forwardM-bM-^@M-^] (request
218 forwardings without command execution), M-bM-^@M-^\cancelM-bM-^@M-^] (cancel
219 forwardings), M-bM-^@M-^\exitM-bM-^@M-^] (request the master to exit), and M-bM-^@M-^\stopM-bM-^@M-^]
220 (request the master to stop accepting further multiplexing
224 Can be used to give options in the format used in the
225 configuration file. This is useful for specifying options for
226 which there is no separate command-line flag. For full details
227 of the options listed below, and their possible values, see
235 CanonicalizeFallbackLocal
238 CanonicalizePermittedCNAMEs
240 ChallengeResponseAuthentication
261 GSSAPIDelegateCredentials
264 HostbasedAuthentication
274 KbdInteractiveAuthentication
275 KbdInteractiveDevices
282 NoHostAuthenticationForLocalhost
283 NumberOfPasswordPrompts
284 PasswordAuthentication
288 PreferredAuthentications
292 PubkeyAcceptedKeyTypes
302 StreamLocalBindUnlink
303 StrictHostKeyChecking
316 Port to connect to on the remote host. This can be specified on
317 a per-host basis in the configuration file.
320 Queries ssh for the algorithms supported for the specified
321 version 2. The available features are: cipher (supported
322 symmetric ciphers), cipher-auth (supported symmetric ciphers that
323 support authenticated encryption), mac (supported message
324 integrity codes), kex (key exchange algorithms), key (key types),
325 key-cert (certificate key types), key-plain (non-certificate key
326 types), and protocol-version (supported SSH protocol versions).
328 -q Quiet mode. Causes most warning and diagnostic messages to be
331 -R [bind_address:]port:host:hostport
332 -R [bind_address:]port:local_socket
333 -R remote_socket:host:hostport
334 -R remote_socket:local_socket
335 -R [bind_address:]port
336 Specifies that connections to the given TCP port or Unix socket
337 on the remote (server) host are to be forwarded to the local
340 This works by allocating a socket to listen to either a TCP port
341 or to a Unix socket on the remote side. Whenever a connection is
342 made to this port or Unix socket, the connection is forwarded
343 over the secure channel, and a connection is made from the local
344 machine to either an explicit destination specified by host port
345 hostport, or local_socket, or, if no explicit destination was
346 specified, ssh will act as a SOCKS 4/5 proxy and forward
347 connections to the destinations requested by the remote SOCKS
350 Port forwardings can also be specified in the configuration file.
351 Privileged ports can be forwarded only when logging in as root on
352 the remote machine. IPv6 addresses can be specified by enclosing
353 the address in square brackets.
355 By default, TCP listening sockets on the server will be bound to
356 the loopback interface only. This may be overridden by
357 specifying a bind_address. An empty bind_address, or the address
358 M-bM-^@M-^X*M-bM-^@M-^Y, indicates that the remote socket should listen on all
359 interfaces. Specifying a remote bind_address will only succeed
360 if the server's GatewayPorts option is enabled (see
363 If the port argument is M-bM-^@M-^X0M-bM-^@M-^Y, the listen port will be dynamically
364 allocated on the server and reported to the client at run time.
365 When used together with -O forward the allocated port will be
366 printed to the standard output.
369 Specifies the location of a control socket for connection
370 sharing, or the string M-bM-^@M-^\noneM-bM-^@M-^] to disable connection sharing.
371 Refer to the description of ControlPath and ControlMaster in
372 ssh_config(5) for details.
374 -s May be used to request invocation of a subsystem on the remote
375 system. Subsystems facilitate the use of SSH as a secure
376 transport for other applications (e.g. sftp(1)). The subsystem
377 is specified as the remote command.
379 -T Disable pseudo-terminal allocation.
381 -t Force pseudo-terminal allocation. This can be used to execute
382 arbitrary screen-based programs on a remote machine, which can be
383 very useful, e.g. when implementing menu services. Multiple -t
384 options force tty allocation, even if ssh has no local tty.
386 -V Display the version number and exit.
388 -v Verbose mode. Causes ssh to print debugging messages about its
389 progress. This is helpful in debugging connection,
390 authentication, and configuration problems. Multiple -v options
391 increase the verbosity. The maximum is 3.
394 Requests that standard input and output on the client be
395 forwarded to host on port over the secure channel. Implies -N,
396 -T, ExitOnForwardFailure and ClearAllForwardings, though these
397 can be overridden in the configuration file or using -o command
400 -w local_tun[:remote_tun]
401 Requests tunnel device forwarding with the specified tun(4)
402 devices between the client (local_tun) and the server
405 The devices may be specified by numerical ID or the keyword
406 M-bM-^@M-^\anyM-bM-^@M-^], which uses the next available tunnel device. If
407 remote_tun is not specified, it defaults to M-bM-^@M-^\anyM-bM-^@M-^]. See also the
408 Tunnel and TunnelDevice directives in ssh_config(5). If the
409 Tunnel directive is unset, it is set to the default tunnel mode,
410 which is M-bM-^@M-^\point-to-pointM-bM-^@M-^].
412 -X Enables X11 forwarding. This can also be specified on a per-host
413 basis in a configuration file.
415 X11 forwarding should be enabled with caution. Users with the
416 ability to bypass file permissions on the remote host (for the
417 user's X authorization database) can access the local X11 display
418 through the forwarded connection. An attacker may then be able
419 to perform activities such as keystroke monitoring.
421 For this reason, X11 forwarding is subjected to X11 SECURITY
422 extension restrictions by default. Please refer to the ssh -Y
423 option and the ForwardX11Trusted directive in ssh_config(5) for
426 -x Disables X11 forwarding.
428 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
429 subjected to the X11 SECURITY extension controls.
431 -y Send log information using the syslog(3) system module. By
432 default this information is sent to stderr.
434 ssh may additionally obtain configuration data from a per-user
435 configuration file and a system-wide configuration file. The file format
436 and configuration options are described in ssh_config(5).
439 The OpenSSH SSH client supports SSH protocol 2.
441 The methods available for authentication are: GSSAPI-based
442 authentication, host-based authentication, public key authentication,
443 challenge-response authentication, and password authentication.
444 Authentication methods are tried in the order specified above, though
445 PreferredAuthentications can be used to change the default order.
447 Host-based authentication works as follows: If the machine the user logs
448 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote
449 machine, and the user names are the same on both sides, or if the files
450 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote
451 machine and contain a line containing the name of the client machine and
452 the name of the user on that machine, the user is considered for login.
453 Additionally, the server must be able to verify the client's host key
454 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts,
455 below) for login to be permitted. This authentication method closes
456 security holes due to IP spoofing, DNS spoofing, and routing spoofing.
457 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the
458 rlogin/rsh protocol in general, are inherently insecure and should be
459 disabled if security is desired.]
461 Public key authentication works as follows: The scheme is based on
462 public-key cryptography, using cryptosystems where encryption and
463 decryption are done using separate keys, and it is unfeasible to derive
464 the decryption key from the encryption key. The idea is that each user
465 creates a public/private key pair for authentication purposes. The
466 server knows the public key, and only the user knows the private key.
467 ssh implements public key authentication protocol automatically, using
468 one of the DSA, ECDSA, Ed25519 or RSA algorithms. The HISTORY section of
469 ssl(8) contains a brief discussion of the DSA and RSA algorithms.
471 The file ~/.ssh/authorized_keys lists the public keys that are permitted
472 for logging in. When the user logs in, the ssh program tells the server
473 which key pair it would like to use for authentication. The client
474 proves that it has access to the private key and the server checks that
475 the corresponding public key is authorized to accept the account.
477 The server may inform the client of errors that prevented public key
478 authentication from succeeding after authentication completes using a
479 different method. These may be viewed by increasing the LogLevel to
480 DEBUG or higher (e.g. by using the -v flag).
482 The user creates his/her key pair by running ssh-keygen(1). This stores
483 the private key in ~/.ssh/id_dsa (DSA), ~/.ssh/id_ecdsa (ECDSA),
484 ~/.ssh/id_ed25519 (Ed25519), or ~/.ssh/id_rsa (RSA) and stores the public
485 key in ~/.ssh/id_dsa.pub (DSA), ~/.ssh/id_ecdsa.pub (ECDSA),
486 ~/.ssh/id_ed25519.pub (Ed25519), or ~/.ssh/id_rsa.pub (RSA) in the user's
487 home directory. The user should then copy the public key to
488 ~/.ssh/authorized_keys in his/her home directory on the remote machine.
489 The authorized_keys file corresponds to the conventional ~/.rhosts file,
490 and has one key per line, though the lines can be very long. After this,
491 the user can log in without giving the password.
493 A variation on public key authentication is available in the form of
494 certificate authentication: instead of a set of public/private keys,
495 signed certificates are used. This has the advantage that a single
496 trusted certification authority can be used in place of many
497 public/private keys. See the CERTIFICATES section of ssh-keygen(1) for
500 The most convenient way to use public key or certificate authentication
501 may be with an authentication agent. See ssh-agent(1) and (optionally)
502 the AddKeysToAgent directive in ssh_config(5) for more information.
504 Challenge-response authentication works as follows: The server sends an
505 arbitrary "challenge" text, and prompts for a response. Examples of
506 challenge-response authentication include BSD Authentication (see
507 login.conf(5)) and PAM (some non-OpenBSD systems).
509 Finally, if other authentication methods fail, ssh prompts the user for a
510 password. The password is sent to the remote host for checking; however,
511 since all communications are encrypted, the password cannot be seen by
512 someone listening on the network.
514 ssh automatically maintains and checks a database containing
515 identification for all hosts it has ever been used with. Host keys are
516 stored in ~/.ssh/known_hosts in the user's home directory. Additionally,
517 the file /etc/ssh/ssh_known_hosts is automatically checked for known
518 hosts. Any new hosts are automatically added to the user's file. If a
519 host's identification ever changes, ssh warns about this and disables
520 password authentication to prevent server spoofing or man-in-the-middle
521 attacks, which could otherwise be used to circumvent the encryption. The
522 StrictHostKeyChecking option can be used to control logins to machines
523 whose host key is not known or has changed.
525 When the user's identity has been accepted by the server, the server
526 either executes the given command in a non-interactive session or, if no
527 command has been specified, logs into the machine and gives the user a
528 normal shell as an interactive session. All communication with the
529 remote command or shell will be automatically encrypted.
531 If an interactive session is requested ssh by default will only request a
532 pseudo-terminal (pty) for interactive sessions when the client has one.
533 The flags -T and -t can be used to override this behaviour.
535 If a pseudo-terminal has been allocated the user may use the escape
536 characters noted below.
538 If no pseudo-terminal has been allocated, the session is transparent and
539 can be used to reliably transfer binary data. On most systems, setting
540 the escape character to M-bM-^@M-^\noneM-bM-^@M-^] will also make the session transparent
541 even if a tty is used.
543 The session terminates when the command or shell on the remote machine
544 exits and all X11 and TCP connections have been closed.
547 When a pseudo-terminal has been requested, ssh supports a number of
548 functions through the use of an escape character.
550 A single tilde character can be sent as ~~ or by following the tilde by a
551 character other than those described below. The escape character must
552 always follow a newline to be interpreted as special. The escape
553 character can be changed in configuration files using the EscapeChar
554 configuration directive or on the command line by the -e option.
556 The supported escapes (assuming the default M-bM-^@M-^X~M-bM-^@M-^Y) are:
562 ~# List forwarded connections.
564 ~& Background ssh at logout when waiting for forwarded connection /
565 X11 sessions to terminate.
567 ~? Display a list of escape characters.
569 ~B Send a BREAK to the remote system (only useful if the peer
572 ~C Open command line. Currently this allows the addition of port
573 forwardings using the -L, -R and -D options (see above). It also
574 allows the cancellation of existing port-forwardings with
575 -KL[bind_address:]port for local, -KR[bind_address:]port for
576 remote and -KD[bind_address:]port for dynamic port-forwardings.
577 !command allows the user to execute a local command if the
578 PermitLocalCommand option is enabled in ssh_config(5). Basic
579 help is available, using the -h option.
581 ~R Request rekeying of the connection (only useful if the peer
584 ~V Decrease the verbosity (LogLevel) when errors are being written
587 ~v Increase the verbosity (LogLevel) when errors are being written
591 Forwarding of arbitrary TCP connections over the secure channel can be
592 specified either on the command line or in a configuration file. One
593 possible application of TCP forwarding is a secure connection to a mail
594 server; another is going through firewalls.
596 In the example below, we look at encrypting communication between an IRC
597 client and server, even though the IRC server does not directly support
598 encrypted communications. This works as follows: the user connects to
599 the remote host using ssh, specifying a port to be used to forward
600 connections to the remote server. After that it is possible to start the
601 service which is to be encrypted on the client machine, connecting to the
602 same local port, and ssh will encrypt and forward the connection.
604 The following example tunnels an IRC session from client machine
605 M-bM-^@M-^\127.0.0.1M-bM-^@M-^] (localhost) to remote server M-bM-^@M-^\server.example.comM-bM-^@M-^]:
607 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
608 $ irc -c '#users' -p 1234 pinky 127.0.0.1
610 This tunnels a connection to IRC server M-bM-^@M-^\server.example.comM-bM-^@M-^], joining
611 channel M-bM-^@M-^\#usersM-bM-^@M-^], nickname M-bM-^@M-^\pinkyM-bM-^@M-^], using port 1234. It doesn't matter
612 which port is used, as long as it's greater than 1023 (remember, only
613 root can open sockets on privileged ports) and doesn't conflict with any
614 ports already in use. The connection is forwarded to port 6667 on the
615 remote server, since that's the standard port for IRC services.
617 The -f option backgrounds ssh and the remote command M-bM-^@M-^\sleep 10M-bM-^@M-^] is
618 specified to allow an amount of time (10 seconds, in the example) to
619 start the service which is to be tunnelled. If no connections are made
620 within the time specified, ssh will exit.
623 If the ForwardX11 variable is set to M-bM-^@M-^\yesM-bM-^@M-^] (or see the description of the
624 -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
625 environment variable is set), the connection to the X11 display is
626 automatically forwarded to the remote side in such a way that any X11
627 programs started from the shell (or command) will go through the
628 encrypted channel, and the connection to the real X server will be made
629 from the local machine. The user should not manually set DISPLAY.
630 Forwarding of X11 connections can be configured on the command line or in
633 The DISPLAY value set by ssh will point to the server machine, but with a
634 display number greater than zero. This is normal, and happens because
635 ssh creates a M-bM-^@M-^\proxyM-bM-^@M-^] X server on the server machine for forwarding the
636 connections over the encrypted channel.
638 ssh will also automatically set up Xauthority data on the server machine.
639 For this purpose, it will generate a random authorization cookie, store
640 it in Xauthority on the server, and verify that any forwarded connections
641 carry this cookie and replace it by the real cookie when the connection
642 is opened. The real authentication cookie is never sent to the server
643 machine (and no cookies are sent in the plain).
645 If the ForwardAgent variable is set to M-bM-^@M-^\yesM-bM-^@M-^] (or see the description of
646 the -A and -a options above) and the user is using an authentication
647 agent, the connection to the agent is automatically forwarded to the
651 When connecting to a server for the first time, a fingerprint of the
652 server's public key is presented to the user (unless the option
653 StrictHostKeyChecking has been disabled). Fingerprints can be determined
656 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
658 If the fingerprint is already known, it can be matched and the key can be
659 accepted or rejected. If only legacy (MD5) fingerprints for the server
660 are available, the ssh-keygen(1) -E option may be used to downgrade the
661 fingerprint algorithm to match.
663 Because of the difficulty of comparing host keys just by looking at
664 fingerprint strings, there is also support to compare host keys visually,
665 using random art. By setting the VisualHostKey option to M-bM-^@M-^\yesM-bM-^@M-^], a small
666 ASCII graphic gets displayed on every login to a server, no matter if the
667 session itself is interactive or not. By learning the pattern a known
668 server produces, a user can easily find out that the host key has changed
669 when a completely different pattern is displayed. Because these patterns
670 are not unambiguous however, a pattern that looks similar to the pattern
671 remembered only gives a good probability that the host key is the same,
672 not guaranteed proof.
674 To get a listing of the fingerprints along with their random art for all
675 known hosts, the following command line can be used:
677 $ ssh-keygen -lv -f ~/.ssh/known_hosts
679 If the fingerprint is unknown, an alternative method of verification is
680 available: SSH fingerprints verified by DNS. An additional resource
681 record (RR), SSHFP, is added to a zonefile and the connecting client is
682 able to match the fingerprint with that of the key presented.
684 In this example, we are connecting a client to a server,
685 M-bM-^@M-^\host.example.comM-bM-^@M-^]. The SSHFP resource records should first be added to
686 the zonefile for host.example.com:
688 $ ssh-keygen -r host.example.com.
690 The output lines will have to be added to the zonefile. To check that
691 the zone is answering fingerprint queries:
693 $ dig -t SSHFP host.example.com
695 Finally the client connects:
697 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
699 Matching host key fingerprint found in DNS.
700 Are you sure you want to continue connecting (yes/no)?
702 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
704 SSH-BASED VIRTUAL PRIVATE NETWORKS
705 ssh contains support for Virtual Private Network (VPN) tunnelling using
706 the tun(4) network pseudo-device, allowing two networks to be joined
707 securely. The sshd_config(5) configuration option PermitTunnel controls
708 whether the server supports this, and at what level (layer 2 or 3
711 The following example would connect client network 10.0.50.0/24 with
712 remote network 10.0.99.0/24 using a point-to-point connection from
713 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway
714 to the remote network, at 192.168.1.15, allows it.
718 # ssh -f -w 0:1 192.168.1.15 true
719 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
720 # route add 10.0.99.0/24 10.1.1.2
724 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
725 # route add 10.0.50.0/24 10.1.1.1
727 Client access may be more finely tuned via the /root/.ssh/authorized_keys
728 file (see below) and the PermitRootLogin server option. The following
729 entry would permit connections on tun(4) device 1 from user M-bM-^@M-^\janeM-bM-^@M-^] and on
730 tun device 2 from user M-bM-^@M-^\johnM-bM-^@M-^], if PermitRootLogin is set to
731 M-bM-^@M-^\forced-commands-onlyM-bM-^@M-^]:
733 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
734 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
736 Since an SSH-based setup entails a fair amount of overhead, it may be
737 more suited to temporary setups, such as for wireless VPNs. More
738 permanent VPNs are better provided by tools such as ipsecctl(8) and
742 ssh will normally set the following environment variables:
744 DISPLAY The DISPLAY variable indicates the location of the
745 X11 server. It is automatically set by ssh to
746 point to a value of the form M-bM-^@M-^\hostname:nM-bM-^@M-^], where
747 M-bM-^@M-^\hostnameM-bM-^@M-^] indicates the host where the shell runs,
748 and M-bM-^@M-^XnM-bM-^@M-^Y is an integer M-bM-^IM-% 1. ssh uses this special
749 value to forward X11 connections over the secure
750 channel. The user should normally not set DISPLAY
751 explicitly, as that will render the X11 connection
752 insecure (and will require the user to manually
753 copy any required authorization cookies).
755 HOME Set to the path of the user's home directory.
757 LOGNAME Synonym for USER; set for compatibility with
758 systems that use this variable.
760 MAIL Set to the path of the user's mailbox.
762 PATH Set to the default PATH, as specified when
765 SSH_ASKPASS If ssh needs a passphrase, it will read the
766 passphrase from the current terminal if it was run
767 from a terminal. If ssh does not have a terminal
768 associated with it but DISPLAY and SSH_ASKPASS are
769 set, it will execute the program specified by
770 SSH_ASKPASS and open an X11 window to read the
771 passphrase. This is particularly useful when
772 calling ssh from a .xsession or related script.
773 (Note that on some machines it may be necessary to
774 redirect the input from /dev/null to make this
777 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
778 communicate with the agent.
780 SSH_CONNECTION Identifies the client and server ends of the
781 connection. The variable contains four space-
782 separated values: client IP address, client port
783 number, server IP address, and server port number.
785 SSH_ORIGINAL_COMMAND This variable contains the original command line if
786 a forced command is executed. It can be used to
787 extract the original arguments.
789 SSH_TTY This is set to the name of the tty (path to the
790 device) associated with the current shell or
791 command. If the current session has no tty, this
794 SSH_TUNNEL Optionally set by sshd(8) to contain the interface
795 names assigned if tunnel forwarding was requested
798 SSH_USER_AUTH Optionally set by sshd(8), this variable may
799 contain a pathname to a file that lists the
800 authentication methods successfully used when the
801 session was established, including any public keys
804 TZ This variable is set to indicate the present time
805 zone if it was set when the daemon was started
806 (i.e. the daemon passes the value on to new
809 USER Set to the name of the user logging in.
811 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
812 M-bM-^@M-^\VARNAME=valueM-bM-^@M-^] to the environment if the file exists and users are
813 allowed to change their environment. For more information, see the
814 PermitUserEnvironment option in sshd_config(5).
818 This file is used for host-based authentication (see above). On
819 some machines this file may need to be world-readable if the
820 user's home directory is on an NFS partition, because sshd(8)
821 reads it as root. Additionally, this file must be owned by the
822 user, and must not have write permissions for anyone else. The
823 recommended permission for most machines is read/write for the
824 user, and not accessible by others.
827 This file is used in exactly the same way as .rhosts, but allows
828 host-based authentication without permitting login with
832 This directory is the default location for all user-specific
833 configuration and authentication information. There is no
834 general requirement to keep the entire contents of this directory
835 secret, but the recommended permissions are read/write/execute
836 for the user, and not accessible by others.
838 ~/.ssh/authorized_keys
839 Lists the public keys (DSA, ECDSA, Ed25519, RSA) that can be used
840 for logging in as this user. The format of this file is
841 described in the sshd(8) manual page. This file is not highly
842 sensitive, but the recommended permissions are read/write for the
843 user, and not accessible by others.
846 This is the per-user configuration file. The file format and
847 configuration options are described in ssh_config(5). Because of
848 the potential for abuse, this file must have strict permissions:
849 read/write for the user, and not writable by others.
852 Contains additional definitions for environment variables; see
859 Contains the private key for authentication. These files contain
860 sensitive data and should be readable by the user but not
861 accessible by others (read/write/execute). ssh will simply
862 ignore a private key file if it is accessible by others. It is
863 possible to specify a passphrase when generating the key which
864 will be used to encrypt the sensitive part of this file using
869 ~/.ssh/id_ed25519.pub
871 Contains the public key for authentication. These files are not
872 sensitive and can (but need not) be readable by anyone.
875 Contains a list of host keys for all hosts the user has logged
876 into that are not already in the systemwide list of known host
877 keys. See sshd(8) for further details of the format of this
881 Commands in this file are executed by ssh when the user logs in,
882 just before the user's shell (or command) is started. See the
883 sshd(8) manual page for more information.
886 This file is for host-based authentication (see above). It
887 should only be writable by root.
890 This file is used in exactly the same way as hosts.equiv, but
891 allows host-based authentication without permitting login with
895 Systemwide configuration file. The file format and configuration
896 options are described in ssh_config(5).
898 /etc/ssh/ssh_host_key
899 /etc/ssh/ssh_host_dsa_key
900 /etc/ssh/ssh_host_ecdsa_key
901 /etc/ssh/ssh_host_ed25519_key
902 /etc/ssh/ssh_host_rsa_key
903 These files contain the private parts of the host keys and are
904 used for host-based authentication.
906 /etc/ssh/ssh_known_hosts
907 Systemwide list of known host keys. This file should be prepared
908 by the system administrator to contain the public host keys of
909 all machines in the organization. It should be world-readable.
910 See sshd(8) for further details of the format of this file.
913 Commands in this file are executed by ssh when the user logs in,
914 just before the user's shell (or command) is started. See the
915 sshd(8) manual page for more information.
918 ssh exits with the exit status of the remote command or with 255 if an
922 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
923 tun(4), ssh_config(5), ssh-keysign(8), sshd(8)
926 S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
927 Numbers, RFC 4250, January 2006.
929 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture,
930 RFC 4251, January 2006.
932 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
933 RFC 4252, January 2006.
935 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer
936 Protocol, RFC 4253, January 2006.
938 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
941 J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
942 (SSH) Key Fingerprints, RFC 4255, January 2006.
944 F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
945 Secure Shell Protocol (SSH), RFC 4256, January 2006.
947 J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
948 Extension, RFC 4335, January 2006.
950 M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
951 Layer Encryption Modes, RFC 4344, January 2006.
953 B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
954 Layer Protocol, RFC 4345, January 2006.
956 M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
957 the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
959 J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File
960 Format, RFC 4716, November 2006.
962 D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the
963 Secure Shell Transport Layer, RFC 5656, December 2009.
965 A. Perrig and D. Song, Hash Visualization: a New Technique to improve
966 Real-World Security, 1999, International Workshop on Cryptographic
967 Techniques and E-Commerce (CrypTEC '99).
970 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
971 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
972 de Raadt and Dug Song removed many bugs, re-added newer features and
973 created OpenSSH. Markus Friedl contributed the support for SSH protocol
974 versions 1.5 and 2.0.
976 OpenBSD 6.2 February 23, 2018 OpenBSD 6.2
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