1 SSH(1) OpenBSD Reference Manual SSH(1)
4 ssh - OpenSSH SSH client (remote login program)
7 ssh [-1246AaCfgKkMNnqsTtVvXxYy] [-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]
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 and arbitrary
21 TCP ports can also be forwarded over the secure channel.
23 ssh connects and logs into the specified hostname (with optional user
24 name). The user must prove his/her identity to the remote machine using
25 one of several methods depending on the protocol version used (see
28 If command is specified, it is executed on the remote host instead of a
31 The options are as follows:
33 -1 Forces ssh to try protocol version 1 only.
35 -2 Forces ssh to try protocol version 2 only.
37 -4 Forces ssh to use IPv4 addresses only.
39 -6 Forces ssh to use IPv6 addresses only.
41 -A Enables forwarding of the authentication agent connection. This
42 can also be specified on a per-host basis in a configuration
45 Agent forwarding should be enabled with caution. Users with the
46 ability to bypass file permissions on the remote host (for the
47 agent's UNIX-domain socket) can access the local agent through
48 the forwarded connection. An attacker cannot obtain key material
49 from the agent, however they can perform operations on the keys
50 that enable them to authenticate using the identities loaded into
53 -a Disables forwarding of the authentication agent connection.
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 and TCP connections). The
62 compression algorithm is the same used by gzip(1), and the
63 ``level'' can be controlled by the CompressionLevel option for
64 protocol version 1. Compression is desirable on modem lines and
65 other slow connections, but will only slow down things on fast
66 networks. The default value can be set on a host-by-host basis
67 in the configuration files; see the Compression option.
70 Selects the cipher specification for encrypting the session.
72 Protocol version 1 allows specification of a single cipher. The
73 supported values are ``3des'', ``blowfish'', and ``des''. 3des
74 (triple-des) is an encrypt-decrypt-encrypt triple with three
75 different keys. It is believed to be secure. blowfish is a fast
76 block cipher; it appears very secure and is much faster than
77 3des. des is only supported in the ssh client for
78 interoperability with legacy protocol 1 implementations that do
79 not support the 3des cipher. Its use is strongly discouraged due
80 to cryptographic weaknesses. The default is ``3des''.
82 For protocol version 2, cipher_spec is a comma-separated list of
83 ciphers listed in order of preference. See the Ciphers keyword
84 in ssh_config(5) for more information.
86 -D [bind_address:]port
87 Specifies a local ``dynamic'' application-level port forwarding.
88 This works by allocating a socket to listen to port on the local
89 side, optionally bound to the specified bind_address. Whenever a
90 connection is made to this port, the connection is forwarded over
91 the secure channel, and the application protocol is then used to
92 determine where to connect to from the remote machine. Currently
93 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
94 as a SOCKS server. Only root can forward privileged ports.
95 Dynamic port forwardings can also be specified in the
98 IPv6 addresses can be specified by enclosing the address in
99 square brackets. Only the superuser can forward privileged
100 ports. By default, the local port is bound in accordance with
101 the GatewayPorts setting. However, an explicit bind_address may
102 be used to bind the connection to a specific address. The
103 bind_address of ``localhost'' indicates that the listening port
104 be bound for local use only, while an empty address or `*'
105 indicates that the port should be available from all interfaces.
108 Append debug logs to log_file instead of standard error.
111 Sets the escape character for sessions with a pty (default: `~').
112 The escape character is only recognized at the beginning of a
113 line. The escape character followed by a dot (`.') closes the
114 connection; followed by control-Z suspends the connection; and
115 followed by itself sends the escape character once. Setting the
116 character to ``none'' disables any escapes and makes the session
120 Specifies an alternative per-user configuration file. If a
121 configuration file is given on the command line, the system-wide
122 configuration file (/etc/ssh/ssh_config) will be ignored. The
123 default for the per-user configuration file is ~/.ssh/config.
125 -f Requests ssh to go to background just before command execution.
126 This is useful if ssh is going to ask for passwords or
127 passphrases, but the user wants it in the background. This
128 implies -n. The recommended way to start X11 programs at a
129 remote site is with something like ssh -f host xterm.
131 If the ExitOnForwardFailure configuration option is set to
132 ``yes'', then a client started with -f will wait for all remote
133 port forwards to be successfully established before placing
134 itself in the background.
136 -g Allows remote hosts to connect to local forwarded ports.
139 Specify the PKCS#11 shared library ssh should use to communicate
140 with a PKCS#11 token providing the user's private RSA key.
143 Selects a file from which the identity (private key) for public
144 key authentication is read. The default is ~/.ssh/identity for
145 protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa,
146 ~/.ssh/id_ed25519 and ~/.ssh/id_rsa for protocol version 2.
147 Identity files may also be specified on a per-host basis in the
148 configuration file. It is possible to have multiple -i options
149 (and multiple identities specified in configuration files). ssh
150 will also try to load certificate information from the filename
151 obtained by appending -cert.pub to identity filenames.
153 -K Enables GSSAPI-based authentication and forwarding (delegation)
154 of GSSAPI credentials to the server.
156 -k Disables forwarding (delegation) of GSSAPI credentials to the
159 -L [bind_address:]port:host:hostport
160 Specifies that the given port on the local (client) host is to be
161 forwarded to the given host and port on the remote side. This
162 works by allocating a socket to listen to port on the local side,
163 optionally bound to the specified bind_address. Whenever a
164 connection is made to this port, the connection is forwarded over
165 the secure channel, and a connection is made to host port
166 hostport from the remote machine. Port forwardings can also be
167 specified in the configuration file. IPv6 addresses can be
168 specified by enclosing the address in square brackets. Only the
169 superuser can forward privileged ports. By default, the local
170 port is bound in accordance with the GatewayPorts setting.
171 However, an explicit bind_address may be used to bind the
172 connection to a specific address. The bind_address of
173 ``localhost'' indicates that the listening port be bound for
174 local use only, while an empty address or `*' indicates that the
175 port should be available from all interfaces.
178 Specifies the user to log in as on the remote machine. This also
179 may be specified on a per-host basis in the configuration file.
181 -M Places the ssh client into ``master'' mode for connection
182 sharing. Multiple -M options places ssh into ``master'' mode
183 with confirmation required before slave connections are accepted.
184 Refer to the description of ControlMaster in ssh_config(5) for
188 Additionally, for protocol version 2 a comma-separated list of
189 MAC (message authentication code) algorithms can be specified in
190 order of preference. See the MACs keyword for more information.
192 -N Do not execute a remote command. This is useful for just
193 forwarding ports (protocol version 2 only).
195 -n Redirects stdin from /dev/null (actually, prevents reading from
196 stdin). This must be used when ssh is run in the background. A
197 common trick is to use this to run X11 programs on a remote
198 machine. For example, ssh -n shadows.cs.hut.fi emacs & will
199 start an emacs on shadows.cs.hut.fi, and the X11 connection will
200 be automatically forwarded over an encrypted channel. The ssh
201 program will be put in the background. (This does not work if
202 ssh needs to ask for a password or passphrase; see also the -f
206 Control an active connection multiplexing master process. When
207 the -O option is specified, the ctl_cmd argument is interpreted
208 and passed to the master process. Valid commands are: ``check''
209 (check that the master process is running), ``forward'' (request
210 forwardings without command execution), ``cancel'' (cancel
211 forwardings), ``exit'' (request the master to exit), and ``stop''
212 (request the master to stop accepting further multiplexing
216 Can be used to give options in the format used in the
217 configuration file. This is useful for specifying options for
218 which there is no separate command-line flag. For full details
219 of the options listed below, and their possible values, see
226 CanonicalizeFallbackLocal
229 CanonicalizePermittedCNAMEs
230 ChallengeResponseAuthentication
252 GSSAPIDelegateCredentials
255 HostbasedAuthentication
262 KbdInteractiveAuthentication
263 KbdInteractiveDevices
270 NoHostAuthenticationForLocalhost
271 NumberOfPasswordPrompts
272 PasswordAuthentication
276 PreferredAuthentications
284 RhostsRSAAuthentication
289 StrictHostKeyChecking
301 Port to connect to on the remote host. This can be specified on
302 a per-host basis in the configuration file.
304 -Q cipher | cipher-auth | mac | kex | key
305 Queries ssh for the algorithms supported for the specified
306 version 2. The available features are: cipher (supported
307 symmetric ciphers), cipher-auth (supported symmetric ciphers that
308 support authenticated encryption), mac (supported message
309 integrity codes), kex (key exchange algorithms), key (key types).
311 -q Quiet mode. Causes most warning and diagnostic messages to be
314 -R [bind_address:]port:host:hostport
315 Specifies that the given port on the remote (server) host is to
316 be forwarded to the given host and port on the local side. This
317 works by allocating a socket to listen to port on the remote
318 side, and whenever a connection is made to this port, the
319 connection is forwarded over the secure channel, and a connection
320 is made to host port hostport from the local machine.
322 Port forwardings can also be specified in the configuration file.
323 Privileged ports can be forwarded only when logging in as root on
324 the remote machine. IPv6 addresses can be specified by enclosing
325 the address in square brackets.
327 By default, the listening socket on the server will be bound to
328 the loopback interface only. This may be overridden by
329 specifying a bind_address. An empty bind_address, or the address
330 `*', indicates that the remote socket should listen on all
331 interfaces. Specifying a remote bind_address will only succeed
332 if the server's GatewayPorts option is enabled (see
335 If the port argument is `0', the listen port will be dynamically
336 allocated on the server and reported to the client at run time.
337 When used together with -O forward the allocated port will be
338 printed to the standard output.
341 Specifies the location of a control socket for connection
342 sharing, or the string ``none'' to disable connection sharing.
343 Refer to the description of ControlPath and ControlMaster in
344 ssh_config(5) for details.
346 -s May be used to request invocation of a subsystem on the remote
347 system. Subsystems are a feature of the SSH2 protocol which
348 facilitate the use of SSH as a secure transport for other
349 applications (eg. sftp(1)). The subsystem is specified as the
352 -T Disable pseudo-tty allocation.
354 -t Force pseudo-tty allocation. This can be used to execute
355 arbitrary screen-based programs on a remote machine, which can be
356 very useful, e.g. when implementing menu services. Multiple -t
357 options force tty allocation, even if ssh has no local tty.
359 -V Display the version number and exit.
361 -v Verbose mode. Causes ssh to print debugging messages about its
362 progress. This is helpful in debugging connection,
363 authentication, and configuration problems. Multiple -v options
364 increase the verbosity. The maximum is 3.
367 Requests that standard input and output on the client be
368 forwarded to host on port over the secure channel. Implies -N,
369 -T, ExitOnForwardFailure and ClearAllForwardings. Works with
370 Protocol version 2 only.
372 -w local_tun[:remote_tun]
373 Requests tunnel device forwarding with the specified tun(4)
374 devices between the client (local_tun) and the server
377 The devices may be specified by numerical ID or the keyword
378 ``any'', which uses the next available tunnel device. If
379 remote_tun is not specified, it defaults to ``any''. See also
380 the Tunnel and TunnelDevice directives in ssh_config(5). If the
381 Tunnel directive is unset, it is set to the default tunnel mode,
382 which is ``point-to-point''.
384 -X Enables X11 forwarding. This can also be specified on a per-host
385 basis in a configuration file.
387 X11 forwarding should be enabled with caution. Users with the
388 ability to bypass file permissions on the remote host (for the
389 user's X authorization database) can access the local X11 display
390 through the forwarded connection. An attacker may then be able
391 to perform activities such as keystroke monitoring.
393 For this reason, X11 forwarding is subjected to X11 SECURITY
394 extension restrictions by default. Please refer to the ssh -Y
395 option and the ForwardX11Trusted directive in ssh_config(5) for
398 -x Disables X11 forwarding.
400 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
401 subjected to the X11 SECURITY extension controls.
403 -y Send log information using the syslog(3) system module. By
404 default this information is sent to stderr.
406 ssh may additionally obtain configuration data from a per-user
407 configuration file and a system-wide configuration file. The file format
408 and configuration options are described in ssh_config(5).
411 The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to
412 use protocol 2 only, though this can be changed via the Protocol option
413 in ssh_config(5) or the -1 and -2 options (see above). Both protocols
414 support similar authentication methods, but protocol 2 is the default
415 since it provides additional mechanisms for confidentiality (the traffic
416 is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and
417 integrity (hmac-md5, hmac-sha1, hmac-sha2-256, hmac-sha2-512, umac-64,
418 umac-128, hmac-ripemd160). Protocol 1 lacks a strong mechanism for
419 ensuring the integrity of the connection.
421 The methods available for authentication are: GSSAPI-based
422 authentication, host-based authentication, public key authentication,
423 challenge-response authentication, and password authentication.
424 Authentication methods are tried in the order specified above, though
425 protocol 2 has a configuration option to change the default order:
426 PreferredAuthentications.
428 Host-based authentication works as follows: If the machine the user logs
429 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote
430 machine, and the user names are the same on both sides, or if the files
431 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote
432 machine and contain a line containing the name of the client machine and
433 the name of the user on that machine, the user is considered for login.
434 Additionally, the server must be able to verify the client's host key
435 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts,
436 below) for login to be permitted. This authentication method closes
437 security holes due to IP spoofing, DNS spoofing, and routing spoofing.
438 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the
439 rlogin/rsh protocol in general, are inherently insecure and should be
440 disabled if security is desired.]
442 Public key authentication works as follows: The scheme is based on
443 public-key cryptography, using cryptosystems where encryption and
444 decryption are done using separate keys, and it is unfeasible to derive
445 the decryption key from the encryption key. The idea is that each user
446 creates a public/private key pair for authentication purposes. The
447 server knows the public key, and only the user knows the private key.
448 ssh implements public key authentication protocol automatically, using
449 one of the DSA, ECDSA, ED25519 or RSA algorithms. Protocol 1 is
450 restricted to using only RSA keys, but protocol 2 may use any. The
451 HISTORY section of ssl(8) contains a brief discussion of the DSA and RSA
454 The file ~/.ssh/authorized_keys lists the public keys that are permitted
455 for logging in. When the user logs in, the ssh program tells the server
456 which key pair it would like to use for authentication. The client
457 proves that it has access to the private key and the server checks that
458 the corresponding public key is authorized to accept the account.
460 The user creates his/her key pair by running ssh-keygen(1). This stores
461 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol
462 2 DSA), ~/.ssh/id_ecdsa (protocol 2 ECDSA), ~/.ssh/id_ed25519 (protocol 2
463 ED25519), or ~/.ssh/id_rsa (protocol 2 RSA) and stores the public key in
464 ~/.ssh/identity.pub (protocol 1), ~/.ssh/id_dsa.pub (protocol 2 DSA),
465 ~/.ssh/id_ecdsa.pub (protocol 2 ECDSA), ~/.ssh/id_ed25519.pub (protocol 2
466 ED25519), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home
467 directory. The user should then copy the public key to
468 ~/.ssh/authorized_keys in his/her home directory on the remote machine.
469 The authorized_keys file corresponds to the conventional ~/.rhosts file,
470 and has one key per line, though the lines can be very long. After this,
471 the user can log in without giving the password.
473 A variation on public key authentication is available in the form of
474 certificate authentication: instead of a set of public/private keys,
475 signed certificates are used. This has the advantage that a single
476 trusted certification authority can be used in place of many
477 public/private keys. See the CERTIFICATES section of ssh-keygen(1) for
480 The most convenient way to use public key or certificate authentication
481 may be with an authentication agent. See ssh-agent(1) for more
484 Challenge-response authentication works as follows: The server sends an
485 arbitrary "challenge" text, and prompts for a response. Protocol 2
486 allows multiple challenges and responses; protocol 1 is restricted to
487 just one challenge/response. Examples of challenge-response
488 authentication include BSD Authentication (see login.conf(5)) and PAM
489 (some non-OpenBSD systems).
491 Finally, if other authentication methods fail, ssh prompts the user for a
492 password. The password is sent to the remote host for checking; however,
493 since all communications are encrypted, the password cannot be seen by
494 someone listening on the network.
496 ssh automatically maintains and checks a database containing
497 identification for all hosts it has ever been used with. Host keys are
498 stored in ~/.ssh/known_hosts in the user's home directory. Additionally,
499 the file /etc/ssh/ssh_known_hosts is automatically checked for known
500 hosts. Any new hosts are automatically added to the user's file. If a
501 host's identification ever changes, ssh warns about this and disables
502 password authentication to prevent server spoofing or man-in-the-middle
503 attacks, which could otherwise be used to circumvent the encryption. The
504 StrictHostKeyChecking option can be used to control logins to machines
505 whose host key is not known or has changed.
507 When the user's identity has been accepted by the server, the server
508 either executes the given command, or logs into the machine and gives the
509 user a normal shell on the remote machine. All communication with the
510 remote command or shell will be automatically encrypted.
512 If a pseudo-terminal has been allocated (normal login session), the user
513 may use the escape characters noted below.
515 If no pseudo-tty has been allocated, the session is transparent and can
516 be used to reliably transfer binary data. On most systems, setting the
517 escape character to ``none'' will also make the session transparent even
520 The session terminates when the command or shell on the remote machine
521 exits and all X11 and TCP connections have been closed.
524 When a pseudo-terminal has been requested, ssh supports a number of
525 functions through the use of an escape character.
527 A single tilde character can be sent as ~~ or by following the tilde by a
528 character other than those described below. The escape character must
529 always follow a newline to be interpreted as special. The escape
530 character can be changed in configuration files using the EscapeChar
531 configuration directive or on the command line by the -e option.
533 The supported escapes (assuming the default `~') are:
539 ~# List forwarded connections.
541 ~& Background ssh at logout when waiting for forwarded connection /
542 X11 sessions to terminate.
544 ~? Display a list of escape characters.
546 ~B Send a BREAK to the remote system (only useful for SSH protocol
547 version 2 and if the peer supports it).
549 ~C Open command line. Currently this allows the addition of port
550 forwardings using the -L, -R and -D options (see above). It also
551 allows the cancellation of existing port-forwardings with
552 -KL[bind_address:]port for local, -KR[bind_address:]port for
553 remote and -KD[bind_address:]port for dynamic port-forwardings.
554 !command allows the user to execute a local command if the
555 PermitLocalCommand option is enabled in ssh_config(5). Basic
556 help is available, using the -h option.
558 ~R Request rekeying of the connection (only useful for SSH protocol
559 version 2 and if the peer supports it).
561 ~V Decrease the verbosity (LogLevel) when errors are being written
564 ~v Increase the verbosity (LogLevel) when errors are being written
568 Forwarding of arbitrary TCP connections over the secure channel can be
569 specified either on the command line or in a configuration file. One
570 possible application of TCP forwarding is a secure connection to a mail
571 server; another is going through firewalls.
573 In the example below, we look at encrypting communication between an IRC
574 client and server, even though the IRC server does not directly support
575 encrypted communications. This works as follows: the user connects to
576 the remote host using ssh, specifying a port to be used to forward
577 connections to the remote server. After that it is possible to start the
578 service which is to be encrypted on the client machine, connecting to the
579 same local port, and ssh will encrypt and forward the connection.
581 The following example tunnels an IRC session from client machine
582 ``127.0.0.1'' (localhost) to remote server ``server.example.com'':
584 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
585 $ irc -c '#users' -p 1234 pinky 127.0.0.1
587 This tunnels a connection to IRC server ``server.example.com'', joining
588 channel ``#users'', nickname ``pinky'', using port 1234. It doesn't
589 matter which port is used, as long as it's greater than 1023 (remember,
590 only root can open sockets on privileged ports) and doesn't conflict with
591 any ports already in use. The connection is forwarded to port 6667 on
592 the remote server, since that's the standard port for IRC services.
594 The -f option backgrounds ssh and the remote command ``sleep 10'' is
595 specified to allow an amount of time (10 seconds, in the example) to
596 start the service which is to be tunnelled. If no connections are made
597 within the time specified, ssh will exit.
600 If the ForwardX11 variable is set to ``yes'' (or see the description of
601 the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
602 environment variable is set), the connection to the X11 display is
603 automatically forwarded to the remote side in such a way that any X11
604 programs started from the shell (or command) will go through the
605 encrypted channel, and the connection to the real X server will be made
606 from the local machine. The user should not manually set DISPLAY.
607 Forwarding of X11 connections can be configured on the command line or in
610 The DISPLAY value set by ssh will point to the server machine, but with a
611 display number greater than zero. This is normal, and happens because
612 ssh creates a ``proxy'' X server on the server machine for forwarding the
613 connections over the encrypted channel.
615 ssh will also automatically set up Xauthority data on the server machine.
616 For this purpose, it will generate a random authorization cookie, store
617 it in Xauthority on the server, and verify that any forwarded connections
618 carry this cookie and replace it by the real cookie when the connection
619 is opened. The real authentication cookie is never sent to the server
620 machine (and no cookies are sent in the plain).
622 If the ForwardAgent variable is set to ``yes'' (or see the description of
623 the -A and -a options above) and the user is using an authentication
624 agent, the connection to the agent is automatically forwarded to the
628 When connecting to a server for the first time, a fingerprint of the
629 server's public key is presented to the user (unless the option
630 StrictHostKeyChecking has been disabled). Fingerprints can be determined
633 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
635 If the fingerprint is already known, it can be matched and the key can be
636 accepted or rejected. Because of the difficulty of comparing host keys
637 just by looking at hex strings, there is also support to compare host
638 keys visually, using random art. By setting the VisualHostKey option to
639 ``yes'', a small ASCII graphic gets displayed on every login to a server,
640 no matter if the session itself is interactive or not. By learning the
641 pattern a known server produces, a user can easily find out that the host
642 key has changed when a completely different pattern is displayed.
643 Because these patterns are not unambiguous however, a pattern that looks
644 similar to the pattern remembered only gives a good probability that the
645 host key is the same, not guaranteed proof.
647 To get a listing of the fingerprints along with their random art for all
648 known hosts, the following command line can be used:
650 $ ssh-keygen -lv -f ~/.ssh/known_hosts
652 If the fingerprint is unknown, an alternative method of verification is
653 available: SSH fingerprints verified by DNS. An additional resource
654 record (RR), SSHFP, is added to a zonefile and the connecting client is
655 able to match the fingerprint with that of the key presented.
657 In this example, we are connecting a client to a server,
658 ``host.example.com''. The SSHFP resource records should first be added
659 to the zonefile for host.example.com:
661 $ ssh-keygen -r host.example.com.
663 The output lines will have to be added to the zonefile. To check that
664 the zone is answering fingerprint queries:
666 $ dig -t SSHFP host.example.com
668 Finally the client connects:
670 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
672 Matching host key fingerprint found in DNS.
673 Are you sure you want to continue connecting (yes/no)?
675 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
677 SSH-BASED VIRTUAL PRIVATE NETWORKS
678 ssh contains support for Virtual Private Network (VPN) tunnelling using
679 the tun(4) network pseudo-device, allowing two networks to be joined
680 securely. The sshd_config(5) configuration option PermitTunnel controls
681 whether the server supports this, and at what level (layer 2 or 3
684 The following example would connect client network 10.0.50.0/24 with
685 remote network 10.0.99.0/24 using a point-to-point connection from
686 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway
687 to the remote network, at 192.168.1.15, allows it.
691 # ssh -f -w 0:1 192.168.1.15 true
692 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
693 # route add 10.0.99.0/24 10.1.1.2
697 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
698 # route add 10.0.50.0/24 10.1.1.1
700 Client access may be more finely tuned via the /root/.ssh/authorized_keys
701 file (see below) and the PermitRootLogin server option. The following
702 entry would permit connections on tun(4) device 1 from user ``jane'' and
703 on tun device 2 from user ``john'', if PermitRootLogin is set to
704 ``forced-commands-only'':
706 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
707 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
709 Since an SSH-based setup entails a fair amount of overhead, it may be
710 more suited to temporary setups, such as for wireless VPNs. More
711 permanent VPNs are better provided by tools such as ipsecctl(8) and
715 ssh will normally set the following environment variables:
717 DISPLAY The DISPLAY variable indicates the location of the
718 X11 server. It is automatically set by ssh to
719 point to a value of the form ``hostname:n'', where
720 ``hostname'' indicates the host where the shell
721 runs, and `n' is an integer >= 1. ssh uses this
722 special value to forward X11 connections over the
723 secure channel. The user should normally not set
724 DISPLAY explicitly, as that will render the X11
725 connection insecure (and will require the user to
726 manually copy any required authorization cookies).
728 HOME Set to the path of the user's home directory.
730 LOGNAME Synonym for USER; set for compatibility with
731 systems that use this variable.
733 MAIL Set to the path of the user's mailbox.
735 PATH Set to the default PATH, as specified when
738 SSH_ASKPASS If ssh needs a passphrase, it will read the
739 passphrase from the current terminal if it was run
740 from a terminal. If ssh does not have a terminal
741 associated with it but DISPLAY and SSH_ASKPASS are
742 set, it will execute the program specified by
743 SSH_ASKPASS and open an X11 window to read the
744 passphrase. This is particularly useful when
745 calling ssh from a .xsession or related script.
746 (Note that on some machines it may be necessary to
747 redirect the input from /dev/null to make this
750 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
751 communicate with the agent.
753 SSH_CONNECTION Identifies the client and server ends of the
754 connection. The variable contains four space-
755 separated values: client IP address, client port
756 number, server IP address, and server port number.
758 SSH_ORIGINAL_COMMAND This variable contains the original command line if
759 a forced command is executed. It can be used to
760 extract the original arguments.
762 SSH_TTY This is set to the name of the tty (path to the
763 device) associated with the current shell or
764 command. If the current session has no tty, this
767 TZ This variable is set to indicate the present time
768 zone if it was set when the daemon was started
769 (i.e. the daemon passes the value on to new
772 USER Set to the name of the user logging in.
774 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
775 ``VARNAME=value'' to the environment if the file exists and users are
776 allowed to change their environment. For more information, see the
777 PermitUserEnvironment option in sshd_config(5).
781 This file is used for host-based authentication (see above). On
782 some machines this file may need to be world-readable if the
783 user's home directory is on an NFS partition, because sshd(8)
784 reads it as root. Additionally, this file must be owned by the
785 user, and must not have write permissions for anyone else. The
786 recommended permission for most machines is read/write for the
787 user, and not accessible by others.
790 This file is used in exactly the same way as .rhosts, but allows
791 host-based authentication without permitting login with
795 This directory is the default location for all user-specific
796 configuration and authentication information. There is no
797 general requirement to keep the entire contents of this directory
798 secret, but the recommended permissions are read/write/execute
799 for the user, and not accessible by others.
801 ~/.ssh/authorized_keys
802 Lists the public keys (DSA, ECDSA, ED25519, RSA) that can be used
803 for logging in as this user. The format of this file is
804 described in the sshd(8) manual page. This file is not highly
805 sensitive, but the recommended permissions are read/write for the
806 user, and not accessible by others.
809 This is the per-user configuration file. The file format and
810 configuration options are described in ssh_config(5). Because of
811 the potential for abuse, this file must have strict permissions:
812 read/write for the user, and not writable by others.
815 Contains additional definitions for environment variables; see
823 Contains the private key for authentication. These files contain
824 sensitive data and should be readable by the user but not
825 accessible by others (read/write/execute). ssh will simply
826 ignore a private key file if it is accessible by others. It is
827 possible to specify a passphrase when generating the key which
828 will be used to encrypt the sensitive part of this file using
834 ~/.ssh/id_ed25519.pub
836 Contains the public key for authentication. These files are not
837 sensitive and can (but need not) be readable by anyone.
840 Contains a list of host keys for all hosts the user has logged
841 into that are not already in the systemwide list of known host
842 keys. See sshd(8) for further details of the format of this
846 Commands in this file are executed by ssh when the user logs in,
847 just before the user's shell (or command) is started. See the
848 sshd(8) manual page for more information.
851 This file is for host-based authentication (see above). It
852 should only be writable by root.
855 This file is used in exactly the same way as hosts.equiv, but
856 allows host-based authentication without permitting login with
860 Systemwide configuration file. The file format and configuration
861 options are described in ssh_config(5).
863 /etc/ssh/ssh_host_key
864 /etc/ssh/ssh_host_dsa_key
865 /etc/ssh/ssh_host_ecdsa_key
866 /etc/ssh/ssh_host_ed25519_key
867 /etc/ssh/ssh_host_rsa_key
868 These files contain the private parts of the host keys and are
869 used for host-based authentication. If protocol version 1 is
870 used, ssh must be setuid root, since the host key is readable
871 only by root. For protocol version 2, ssh uses ssh-keysign(8) to
872 access the host keys, eliminating the requirement that ssh be
873 setuid root when host-based authentication is used. By default
874 ssh is not setuid root.
876 /etc/ssh/ssh_known_hosts
877 Systemwide list of known host keys. This file should be prepared
878 by the system administrator to contain the public host keys of
879 all machines in the organization. It should be world-readable.
880 See sshd(8) for further details of the format of this file.
883 Commands in this file are executed by ssh when the user logs in,
884 just before the user's shell (or command) is started. See the
885 sshd(8) manual page for more information.
888 ssh exits with the exit status of the remote command or with 255 if an
892 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
893 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8)
896 S. Lehtinen and C. Lonvick, The Secure Shell (SSH) Protocol Assigned
897 Numbers, RFC 4250, January 2006.
899 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Protocol Architecture,
900 RFC 4251, January 2006.
902 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Authentication Protocol,
903 RFC 4252, January 2006.
905 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Transport Layer
906 Protocol, RFC 4253, January 2006.
908 T. Ylonen and C. Lonvick, The Secure Shell (SSH) Connection Protocol, RFC
911 J. Schlyter and W. Griffin, Using DNS to Securely Publish Secure Shell
912 (SSH) Key Fingerprints, RFC 4255, January 2006.
914 F. Cusack and M. Forssen, Generic Message Exchange Authentication for the
915 Secure Shell Protocol (SSH), RFC 4256, January 2006.
917 J. Galbraith and P. Remaker, The Secure Shell (SSH) Session Channel Break
918 Extension, RFC 4335, January 2006.
920 M. Bellare, T. Kohno, and C. Namprempre, The Secure Shell (SSH) Transport
921 Layer Encryption Modes, RFC 4344, January 2006.
923 B. Harris, Improved Arcfour Modes for the Secure Shell (SSH) Transport
924 Layer Protocol, RFC 4345, January 2006.
926 M. Friedl, N. Provos, and W. Simpson, Diffie-Hellman Group Exchange for
927 the Secure Shell (SSH) Transport Layer Protocol, RFC 4419, March 2006.
929 J. Galbraith and R. Thayer, The Secure Shell (SSH) Public Key File
930 Format, RFC 4716, November 2006.
932 D. Stebila and J. Green, Elliptic Curve Algorithm Integration in the
933 Secure Shell Transport Layer, RFC 5656, December 2009.
935 A. Perrig and D. Song, Hash Visualization: a New Technique to improve
936 Real-World Security, 1999, International Workshop on Cryptographic
937 Techniques and E-Commerce (CrypTEC '99).
940 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
941 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
942 de Raadt and Dug Song removed many bugs, re-added newer features and
943 created OpenSSH. Markus Friedl contributed the support for SSH protocol
944 versions 1.5 and 2.0.
946 OpenBSD 5.5 December 7, 2013 OpenBSD 5.5
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