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 escape_char] [-F configfile] [-I pkcs11]
9 [-i identity_file] [-L [bind_address:]port:host:hostport]
10 [-l login_name] [-m mac_spec] [-O ctl_cmd] [-o option] [-p port]
11 [-R [bind_address:]port:host:hostport] [-S ctl_path] [-W host:port]
12 [-w local_tun[:remote_tun]] [user@]hostname [command]
15 ssh (SSH client) is a program for logging into a remote machine and for
16 executing commands on a remote machine. It is intended to replace rlogin
17 and rsh, and provide secure encrypted communications between two
18 untrusted hosts over an insecure network. X11 connections and arbitrary
19 TCP ports can also be forwarded over the secure channel.
21 ssh connects and logs into the specified hostname (with optional user
22 name). The user must prove his/her identity to the remote machine using
23 one of several methods depending on the protocol version used (see
26 If command is specified, it is executed on the remote host instead of a
29 The options are as follows:
31 -1 Forces ssh to try protocol version 1 only.
33 -2 Forces ssh to try protocol version 2 only.
35 -4 Forces ssh to use IPv4 addresses only.
37 -6 Forces ssh to use IPv6 addresses only.
39 -A Enables forwarding of the authentication agent connection. This
40 can also be specified on a per-host basis in a configuration
43 Agent forwarding should be enabled with caution. Users with the
44 ability to bypass file permissions on the remote host (for the
45 agent's UNIX-domain socket) can access the local agent through
46 the forwarded connection. An attacker cannot obtain key material
47 from the agent, however they can perform operations on the keys
48 that enable them to authenticate using the identities loaded into
51 -a Disables forwarding of the authentication agent connection.
54 Use bind_address on the local machine as the source address of
55 the connection. Only useful on systems with more than one
58 -C Requests compression of all data (including stdin, stdout,
59 stderr, and data for forwarded X11 and TCP connections). The
60 compression algorithm is the same used by gzip(1), and the
61 ``level'' can be controlled by the CompressionLevel option for
62 protocol version 1. Compression is desirable on modem lines and
63 other slow connections, but will only slow down things on fast
64 networks. The default value can be set on a host-by-host basis
65 in the configuration files; see the Compression option.
68 Selects the cipher specification for encrypting the session.
70 Protocol version 1 allows specification of a single cipher. The
71 supported values are ``3des'', ``blowfish'', and ``des''. 3des
72 (triple-des) is an encrypt-decrypt-encrypt triple with three
73 different keys. It is believed to be secure. blowfish is a fast
74 block cipher; it appears very secure and is much faster than
75 3des. des is only supported in the ssh client for
76 interoperability with legacy protocol 1 implementations that do
77 not support the 3des cipher. Its use is strongly discouraged due
78 to cryptographic weaknesses. The default is ``3des''.
80 For protocol version 2, cipher_spec is a comma-separated list of
81 ciphers listed in order of preference. See the Ciphers keyword
82 in ssh_config(5) for more information.
84 -D [bind_address:]port
85 Specifies a local ``dynamic'' application-level port forwarding.
86 This works by allocating a socket to listen to port on the local
87 side, optionally bound to the specified bind_address. Whenever a
88 connection is made to this port, the connection is forwarded over
89 the secure channel, and the application protocol is then used to
90 determine where to connect to from the remote machine. Currently
91 the SOCKS4 and SOCKS5 protocols are supported, and ssh will act
92 as a SOCKS server. Only root can forward privileged ports.
93 Dynamic port forwardings can also be specified in the
96 IPv6 addresses can be specified by enclosing the address in
97 square brackets. Only the superuser can forward privileged
98 ports. By default, the local port is bound in accordance with
99 the GatewayPorts setting. However, an explicit bind_address may
100 be used to bind the connection to a specific address. The
101 bind_address of ``localhost'' indicates that the listening port
102 be bound for local use only, while an empty address or `*'
103 indicates that the port should be available from all interfaces.
106 Sets the escape character for sessions with a pty (default: `~').
107 The escape character is only recognized at the beginning of a
108 line. The escape character followed by a dot (`.') closes the
109 connection; followed by control-Z suspends the connection; and
110 followed by itself sends the escape character once. Setting the
111 character to ``none'' disables any escapes and makes the session
115 Specifies an alternative per-user configuration file. If a
116 configuration file is given on the command line, the system-wide
117 configuration file (/etc/ssh/ssh_config) will be ignored. The
118 default for the per-user configuration file is ~/.ssh/config.
120 -f Requests ssh to go to background just before command execution.
121 This is useful if ssh is going to ask for passwords or
122 passphrases, but the user wants it in the background. This
123 implies -n. The recommended way to start X11 programs at a
124 remote site is with something like ssh -f host xterm.
126 If the ExitOnForwardFailure configuration option is set to
127 ``yes'', then a client started with -f will wait for all remote
128 port forwards to be successfully established before placing
129 itself in the background.
131 -g Allows remote hosts to connect to local forwarded ports.
134 Specify the PKCS#11 shared library ssh should use to communicate
135 with a PKCS#11 token providing the user's private RSA key.
138 Selects a file from which the identity (private key) for public
139 key authentication is read. The default is ~/.ssh/identity for
140 protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa and
141 ~/.ssh/id_rsa for protocol version 2. Identity files may also be
142 specified on a per-host basis in the configuration file. It is
143 possible to have multiple -i options (and multiple identities
144 specified in configuration files). ssh will also try to load
145 certificate information from the filename obtained by appending
146 -cert.pub to identity filenames.
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 Specifies that the given port on the local (client) host is to be
156 forwarded to the given host and port on the remote side. This
157 works by allocating a socket to listen to port on the local side,
158 optionally bound to the specified bind_address. Whenever a
159 connection is made to this port, the connection is forwarded over
160 the secure channel, and a connection is made to host port
161 hostport from the remote machine. Port forwardings can also be
162 specified in the configuration file. IPv6 addresses can be
163 specified by enclosing the address in square brackets. Only the
164 superuser can forward privileged ports. By default, the local
165 port is bound in accordance with the GatewayPorts setting.
166 However, an explicit bind_address may be used to bind the
167 connection to a specific address. The bind_address of
168 ``localhost'' indicates that the listening port be bound for
169 local use only, while an empty address or `*' indicates that the
170 port should be available from all interfaces.
173 Specifies the user to log in as on the remote machine. This also
174 may be specified on a per-host basis in the configuration file.
176 -M Places the ssh client into ``master'' mode for connection
177 sharing. Multiple -M options places ssh into ``master'' mode
178 with confirmation required before slave connections are accepted.
179 Refer to the description of ControlMaster in ssh_config(5) for
183 Additionally, for protocol version 2 a comma-separated list of
184 MAC (message authentication code) algorithms can be specified in
185 order of preference. See the MACs keyword for more information.
187 -N Do not execute a remote command. This is useful for just
188 forwarding ports (protocol version 2 only).
190 -n Redirects stdin from /dev/null (actually, prevents reading from
191 stdin). This must be used when ssh is run in the background. A
192 common trick is to use this to run X11 programs on a remote
193 machine. For example, ssh -n shadows.cs.hut.fi emacs & will
194 start an emacs on shadows.cs.hut.fi, and the X11 connection will
195 be automatically forwarded over an encrypted channel. The ssh
196 program will be put in the background. (This does not work if
197 ssh needs to ask for a password or passphrase; see also the -f
201 Control an active connection multiplexing master process. When
202 the -O option is specified, the ctl_cmd argument is interpreted
203 and passed to the master process. Valid commands are: ``check''
204 (check that the master process is running), ``forward'' (request
205 forwardings without command execution) and ``exit'' (request the
209 Can be used to give options in the format used in the
210 configuration file. This is useful for specifying options for
211 which there is no separate command-line flag. For full details
212 of the options listed below, and their possible values, see
218 ChallengeResponseAuthentication
238 GSSAPIDelegateCredentials
241 HostbasedAuthentication
248 KbdInteractiveDevices
254 NoHostAuthenticationForLocalhost
255 NumberOfPasswordPrompts
256 PasswordAuthentication
260 PreferredAuthentications
266 RhostsRSAAuthentication
271 StrictHostKeyChecking
283 Port to connect to on the remote host. This can be specified on
284 a per-host basis in the configuration file.
286 -q Quiet mode. Causes most warning and diagnostic messages to be
289 -R [bind_address:]port:host:hostport
290 Specifies that the given port on the remote (server) host is to
291 be forwarded to the given host and port on the local side. This
292 works by allocating a socket to listen to port on the remote
293 side, and whenever a connection is made to this port, the
294 connection is forwarded over the secure channel, and a connection
295 is made to host port hostport from the local machine.
297 Port forwardings can also be specified in the configuration file.
298 Privileged ports can be forwarded only when logging in as root on
299 the remote machine. IPv6 addresses can be specified by enclosing
300 the address in square braces.
302 By default, the listening socket on the server will be bound to
303 the loopback interface only. This may be overridden by
304 specifying a bind_address. An empty bind_address, or the address
305 `*', indicates that the remote socket should listen on all
306 interfaces. Specifying a remote bind_address will only succeed
307 if the server's GatewayPorts option is enabled (see
310 If the port argument is `0', the listen port will be dynamically
311 allocated on the server and reported to the client at run time.
312 When used together with -O forward the allocated port will be
313 printed to the standard output.
316 Specifies the location of a control socket for connection
317 sharing, or the string ``none'' to disable connection sharing.
318 Refer to the description of ControlPath and ControlMaster in
319 ssh_config(5) for details.
321 -s May be used to request invocation of a subsystem on the remote
322 system. Subsystems are a feature of the SSH2 protocol which
323 facilitate the use of SSH as a secure transport for other
324 applications (eg. sftp(1)). The subsystem is specified as the
327 -T Disable pseudo-tty allocation.
329 -t Force pseudo-tty allocation. This can be used to execute
330 arbitrary screen-based programs on a remote machine, which can be
331 very useful, e.g. when implementing menu services. Multiple -t
332 options force tty allocation, even if ssh has no local tty.
334 -V Display the version number and exit.
336 -v Verbose mode. Causes ssh to print debugging messages about its
337 progress. This is helpful in debugging connection,
338 authentication, and configuration problems. Multiple -v options
339 increase the verbosity. The maximum is 3.
342 Requests that standard input and output on the client be
343 forwarded to host on port over the secure channel. Implies -N,
344 -T, ExitOnForwardFailure and ClearAllForwardings and works with
345 Protocol version 2 only.
347 -w local_tun[:remote_tun]
348 Requests tunnel device forwarding with the specified tun(4)
349 devices between the client (local_tun) and the server
352 The devices may be specified by numerical ID or the keyword
353 ``any'', which uses the next available tunnel device. If
354 remote_tun is not specified, it defaults to ``any''. See also
355 the Tunnel and TunnelDevice directives in ssh_config(5). If the
356 Tunnel directive is unset, it is set to the default tunnel mode,
357 which is ``point-to-point''.
359 -X Enables X11 forwarding. This can also be specified on a per-host
360 basis in a configuration file.
362 X11 forwarding should be enabled with caution. Users with the
363 ability to bypass file permissions on the remote host (for the
364 user's X authorization database) can access the local X11 display
365 through the forwarded connection. An attacker may then be able
366 to perform activities such as keystroke monitoring.
368 For this reason, X11 forwarding is subjected to X11 SECURITY
369 extension restrictions by default. Please refer to the ssh -Y
370 option and the ForwardX11Trusted directive in ssh_config(5) for
373 -x Disables X11 forwarding.
375 -Y Enables trusted X11 forwarding. Trusted X11 forwardings are not
376 subjected to the X11 SECURITY extension controls.
378 -y Send log information using the syslog(3) system module. By
379 default this information is sent to stderr.
381 ssh may additionally obtain configuration data from a per-user
382 configuration file and a system-wide configuration file. The file format
383 and configuration options are described in ssh_config(5).
386 The OpenSSH SSH client supports SSH protocols 1 and 2. The default is to
387 use protocol 2 only, though this can be changed via the Protocol option
388 in ssh_config(5) or the -1 and -2 options (see above). Both protocols
389 support similar authentication methods, but protocol 2 is the default
390 since it provides additional mechanisms for confidentiality (the traffic
391 is encrypted using AES, 3DES, Blowfish, CAST128, or Arcfour) and
392 integrity (hmac-md5, hmac-sha1, umac-64, hmac-ripemd160). Protocol 1
393 lacks a strong mechanism for ensuring the integrity of the connection.
395 The methods available for authentication are: GSSAPI-based
396 authentication, host-based authentication, public key authentication,
397 challenge-response authentication, and password authentication.
398 Authentication methods are tried in the order specified above, though
399 protocol 2 has a configuration option to change the default order:
400 PreferredAuthentications.
402 Host-based authentication works as follows: If the machine the user logs
403 in from is listed in /etc/hosts.equiv or /etc/shosts.equiv on the remote
404 machine, and the user names are the same on both sides, or if the files
405 ~/.rhosts or ~/.shosts exist in the user's home directory on the remote
406 machine and contain a line containing the name of the client machine and
407 the name of the user on that machine, the user is considered for login.
408 Additionally, the server must be able to verify the client's host key
409 (see the description of /etc/ssh/ssh_known_hosts and ~/.ssh/known_hosts,
410 below) for login to be permitted. This authentication method closes
411 security holes due to IP spoofing, DNS spoofing, and routing spoofing.
412 [Note to the administrator: /etc/hosts.equiv, ~/.rhosts, and the
413 rlogin/rsh protocol in general, are inherently insecure and should be
414 disabled if security is desired.]
416 Public key authentication works as follows: The scheme is based on
417 public-key cryptography, using cryptosystems where encryption and
418 decryption are done using separate keys, and it is unfeasible to derive
419 the decryption key from the encryption key. The idea is that each user
420 creates a public/private key pair for authentication purposes. The
421 server knows the public key, and only the user knows the private key.
422 ssh implements public key authentication protocol automatically, using
423 one of the DSA, ECDSA or RSA algorithms. Protocol 1 is restricted to
424 using only RSA keys, but protocol 2 may use any. The HISTORY section of
425 ssl(8) contains a brief discussion of the DSA and RSA algorithms.
427 The file ~/.ssh/authorized_keys lists the public keys that are permitted
428 for logging in. When the user logs in, the ssh program tells the server
429 which key pair it would like to use for authentication. The client
430 proves that it has access to the private key and the server checks that
431 the corresponding public key is authorized to accept the account.
433 The user creates his/her key pair by running ssh-keygen(1). This stores
434 the private key in ~/.ssh/identity (protocol 1), ~/.ssh/id_dsa (protocol
435 2 DSA), ~/.ssh/id_ecdsa (protocol 2 ECDSA), or ~/.ssh/id_rsa (protocol 2
436 RSA) and stores the public key in ~/.ssh/identity.pub (protocol 1),
437 ~/.ssh/id_dsa.pub (protocol 2 DSA), ~/.ssh/id_ecdsa.pub (protocol 2
438 ECDSA), or ~/.ssh/id_rsa.pub (protocol 2 RSA) in the user's home
439 directory. The user should then copy the public key to
440 ~/.ssh/authorized_keys in his/her home directory on the remote machine.
441 The authorized_keys file corresponds to the conventional ~/.rhosts file,
442 and has one key per line, though the lines can be very long. After this,
443 the user can log in without giving the password.
445 A variation on public key authentication is available in the form of
446 certificate authentication: instead of a set of public/private keys,
447 signed certificates are used. This has the advantage that a single
448 trusted certification authority can be used in place of many
449 public/private keys. See the CERTIFICATES section of ssh-keygen(1) for
452 The most convenient way to use public key or certificate authentication
453 may be with an authentication agent. See ssh-agent(1) for more
456 Challenge-response authentication works as follows: The server sends an
457 arbitrary "challenge" text, and prompts for a response. Protocol 2
458 allows multiple challenges and responses; protocol 1 is restricted to
459 just one challenge/response. Examples of challenge-response
460 authentication include BSD Authentication (see login.conf(5)) and PAM
461 (some non-OpenBSD systems).
463 Finally, if other authentication methods fail, ssh prompts the user for a
464 password. The password is sent to the remote host for checking; however,
465 since all communications are encrypted, the password cannot be seen by
466 someone listening on the network.
468 ssh automatically maintains and checks a database containing
469 identification for all hosts it has ever been used with. Host keys are
470 stored in ~/.ssh/known_hosts in the user's home directory. Additionally,
471 the file /etc/ssh/ssh_known_hosts is automatically checked for known
472 hosts. Any new hosts are automatically added to the user's file. If a
473 host's identification ever changes, ssh warns about this and disables
474 password authentication to prevent server spoofing or man-in-the-middle
475 attacks, which could otherwise be used to circumvent the encryption. The
476 StrictHostKeyChecking option can be used to control logins to machines
477 whose host key is not known or has changed.
479 When the user's identity has been accepted by the server, the server
480 either executes the given command, or logs into the machine and gives the
481 user a normal shell on the remote machine. All communication with the
482 remote command or shell will be automatically encrypted.
484 If a pseudo-terminal has been allocated (normal login session), the user
485 may use the escape characters noted below.
487 If no pseudo-tty has been allocated, the session is transparent and can
488 be used to reliably transfer binary data. On most systems, setting the
489 escape character to ``none'' will also make the session transparent even
492 The session terminates when the command or shell on the remote machine
493 exits and all X11 and TCP connections have been closed.
496 When a pseudo-terminal has been requested, ssh supports a number of
497 functions through the use of an escape character.
499 A single tilde character can be sent as ~~ or by following the tilde by a
500 character other than those described below. The escape character must
501 always follow a newline to be interpreted as special. The escape
502 character can be changed in configuration files using the EscapeChar
503 configuration directive or on the command line by the -e option.
505 The supported escapes (assuming the default `~') are:
511 ~# List forwarded connections.
513 ~& Background ssh at logout when waiting for forwarded connection /
514 X11 sessions to terminate.
516 ~? Display a list of escape characters.
518 ~B Send a BREAK to the remote system (only useful for SSH protocol
519 version 2 and if the peer supports it).
521 ~C Open command line. Currently this allows the addition of port
522 forwardings using the -L, -R and -D options (see above). It also
523 allows the cancellation of existing remote port-forwardings using
524 -KR[bind_address:]port. !command allows the user to execute a
525 local command if the PermitLocalCommand option is enabled in
526 ssh_config(5). Basic help is available, using the -h option.
528 ~R Request rekeying of the connection (only useful for SSH protocol
529 version 2 and if the peer supports it).
532 Forwarding of arbitrary TCP connections over the secure channel can be
533 specified either on the command line or in a configuration file. One
534 possible application of TCP forwarding is a secure connection to a mail
535 server; another is going through firewalls.
537 In the example below, we look at encrypting communication between an IRC
538 client and server, even though the IRC server does not directly support
539 encrypted communications. This works as follows: the user connects to
540 the remote host using ssh, specifying a port to be used to forward
541 connections to the remote server. After that it is possible to start the
542 service which is to be encrypted on the client machine, connecting to the
543 same local port, and ssh will encrypt and forward the connection.
545 The following example tunnels an IRC session from client machine
546 ``127.0.0.1'' (localhost) to remote server ``server.example.com'':
548 $ ssh -f -L 1234:localhost:6667 server.example.com sleep 10
549 $ irc -c '#users' -p 1234 pinky 127.0.0.1
551 This tunnels a connection to IRC server ``server.example.com'', joining
552 channel ``#users'', nickname ``pinky'', using port 1234. It doesn't
553 matter which port is used, as long as it's greater than 1023 (remember,
554 only root can open sockets on privileged ports) and doesn't conflict with
555 any ports already in use. The connection is forwarded to port 6667 on
556 the remote server, since that's the standard port for IRC services.
558 The -f option backgrounds ssh and the remote command ``sleep 10'' is
559 specified to allow an amount of time (10 seconds, in the example) to
560 start the service which is to be tunnelled. If no connections are made
561 within the time specified, ssh will exit.
564 If the ForwardX11 variable is set to ``yes'' (or see the description of
565 the -X, -x, and -Y options above) and the user is using X11 (the DISPLAY
566 environment variable is set), the connection to the X11 display is
567 automatically forwarded to the remote side in such a way that any X11
568 programs started from the shell (or command) will go through the
569 encrypted channel, and the connection to the real X server will be made
570 from the local machine. The user should not manually set DISPLAY.
571 Forwarding of X11 connections can be configured on the command line or in
574 The DISPLAY value set by ssh will point to the server machine, but with a
575 display number greater than zero. This is normal, and happens because
576 ssh creates a ``proxy'' X server on the server machine for forwarding the
577 connections over the encrypted channel.
579 ssh will also automatically set up Xauthority data on the server machine.
580 For this purpose, it will generate a random authorization cookie, store
581 it in Xauthority on the server, and verify that any forwarded connections
582 carry this cookie and replace it by the real cookie when the connection
583 is opened. The real authentication cookie is never sent to the server
584 machine (and no cookies are sent in the plain).
586 If the ForwardAgent variable is set to ``yes'' (or see the description of
587 the -A and -a options above) and the user is using an authentication
588 agent, the connection to the agent is automatically forwarded to the
592 When connecting to a server for the first time, a fingerprint of the
593 server's public key is presented to the user (unless the option
594 StrictHostKeyChecking has been disabled). Fingerprints can be determined
597 $ ssh-keygen -l -f /etc/ssh/ssh_host_rsa_key
599 If the fingerprint is already known, it can be matched and the key can be
600 accepted or rejected. Because of the difficulty of comparing host keys
601 just by looking at hex strings, there is also support to compare host
602 keys visually, using random art. By setting the VisualHostKey option to
603 ``yes'', a small ASCII graphic gets displayed on every login to a server,
604 no matter if the session itself is interactive or not. By learning the
605 pattern a known server produces, a user can easily find out that the host
606 key has changed when a completely different pattern is displayed.
607 Because these patterns are not unambiguous however, a pattern that looks
608 similar to the pattern remembered only gives a good probability that the
609 host key is the same, not guaranteed proof.
611 To get a listing of the fingerprints along with their random art for all
612 known hosts, the following command line can be used:
614 $ ssh-keygen -lv -f ~/.ssh/known_hosts
616 If the fingerprint is unknown, an alternative method of verification is
617 available: SSH fingerprints verified by DNS. An additional resource
618 record (RR), SSHFP, is added to a zonefile and the connecting client is
619 able to match the fingerprint with that of the key presented.
621 In this example, we are connecting a client to a server,
622 ``host.example.com''. The SSHFP resource records should first be added
623 to the zonefile for host.example.com:
625 $ ssh-keygen -r host.example.com.
627 The output lines will have to be added to the zonefile. To check that
628 the zone is answering fingerprint queries:
630 $ dig -t SSHFP host.example.com
632 Finally the client connects:
634 $ ssh -o "VerifyHostKeyDNS ask" host.example.com
636 Matching host key fingerprint found in DNS.
637 Are you sure you want to continue connecting (yes/no)?
639 See the VerifyHostKeyDNS option in ssh_config(5) for more information.
641 SSH-BASED VIRTUAL PRIVATE NETWORKS
642 ssh contains support for Virtual Private Network (VPN) tunnelling using
643 the tun(4) network pseudo-device, allowing two networks to be joined
644 securely. The sshd_config(5) configuration option PermitTunnel controls
645 whether the server supports this, and at what level (layer 2 or 3
648 The following example would connect client network 10.0.50.0/24 with
649 remote network 10.0.99.0/24 using a point-to-point connection from
650 10.1.1.1 to 10.1.1.2, provided that the SSH server running on the gateway
651 to the remote network, at 192.168.1.15, allows it.
655 # ssh -f -w 0:1 192.168.1.15 true
656 # ifconfig tun0 10.1.1.1 10.1.1.2 netmask 255.255.255.252
657 # route add 10.0.99.0/24 10.1.1.2
661 # ifconfig tun1 10.1.1.2 10.1.1.1 netmask 255.255.255.252
662 # route add 10.0.50.0/24 10.1.1.1
664 Client access may be more finely tuned via the /root/.ssh/authorized_keys
665 file (see below) and the PermitRootLogin server option. The following
666 entry would permit connections on tun(4) device 1 from user ``jane'' and
667 on tun device 2 from user ``john'', if PermitRootLogin is set to
668 ``forced-commands-only'':
670 tunnel="1",command="sh /etc/netstart tun1" ssh-rsa ... jane
671 tunnel="2",command="sh /etc/netstart tun2" ssh-rsa ... john
673 Since an SSH-based setup entails a fair amount of overhead, it may be
674 more suited to temporary setups, such as for wireless VPNs. More
675 permanent VPNs are better provided by tools such as ipsecctl(8) and
679 ssh will normally set the following environment variables:
681 DISPLAY The DISPLAY variable indicates the location of the
682 X11 server. It is automatically set by ssh to
683 point to a value of the form ``hostname:n'', where
684 ``hostname'' indicates the host where the shell
685 runs, and `n' is an integer >= 1. ssh uses this
686 special value to forward X11 connections over the
687 secure channel. The user should normally not set
688 DISPLAY explicitly, as that will render the X11
689 connection insecure (and will require the user to
690 manually copy any required authorization cookies).
692 HOME Set to the path of the user's home directory.
694 LOGNAME Synonym for USER; set for compatibility with
695 systems that use this variable.
697 MAIL Set to the path of the user's mailbox.
699 PATH Set to the default PATH, as specified when
702 SSH_ASKPASS If ssh needs a passphrase, it will read the
703 passphrase from the current terminal if it was run
704 from a terminal. If ssh does not have a terminal
705 associated with it but DISPLAY and SSH_ASKPASS are
706 set, it will execute the program specified by
707 SSH_ASKPASS and open an X11 window to read the
708 passphrase. This is particularly useful when
709 calling ssh from a .xsession or related script.
710 (Note that on some machines it may be necessary to
711 redirect the input from /dev/null to make this
714 SSH_AUTH_SOCK Identifies the path of a UNIX-domain socket used to
715 communicate with the agent.
717 SSH_CONNECTION Identifies the client and server ends of the
718 connection. The variable contains four space-
719 separated values: client IP address, client port
720 number, server IP address, and server port number.
722 SSH_ORIGINAL_COMMAND This variable contains the original command line if
723 a forced command is executed. It can be used to
724 extract the original arguments.
726 SSH_TTY This is set to the name of the tty (path to the
727 device) associated with the current shell or
728 command. If the current session has no tty, this
731 TZ This variable is set to indicate the present time
732 zone if it was set when the daemon was started
733 (i.e. the daemon passes the value on to new
736 USER Set to the name of the user logging in.
738 Additionally, ssh reads ~/.ssh/environment, and adds lines of the format
739 ``VARNAME=value'' to the environment if the file exists and users are
740 allowed to change their environment. For more information, see the
741 PermitUserEnvironment option in sshd_config(5).
745 This file is used for host-based authentication (see above). On
746 some machines this file may need to be world-readable if the
747 user's home directory is on an NFS partition, because sshd(8)
748 reads it as root. Additionally, this file must be owned by the
749 user, and must not have write permissions for anyone else. The
750 recommended permission for most machines is read/write for the
751 user, and not accessible by others.
754 This file is used in exactly the same way as .rhosts, but allows
755 host-based authentication without permitting login with
759 This directory is the default location for all user-specific
760 configuration and authentication information. There is no
761 general requirement to keep the entire contents of this directory
762 secret, but the recommended permissions are read/write/execute
763 for the user, and not accessible by others.
765 ~/.ssh/authorized_keys
766 Lists the public keys (DSA/ECDSA/RSA) that can be used for
767 logging in as this user. The format of this file is described in
768 the sshd(8) manual page. This file is not highly sensitive, but
769 the recommended permissions are read/write for the user, and not
770 accessible by others.
773 This is the per-user configuration file. The file format and
774 configuration options are described in ssh_config(5). Because of
775 the potential for abuse, this file must have strict permissions:
776 read/write for the user, and not accessible by others.
779 Contains additional definitions for environment variables; see
786 Contains the private key for authentication. These files contain
787 sensitive data and should be readable by the user but not
788 accessible by others (read/write/execute). ssh will simply
789 ignore a private key file if it is accessible by others. It is
790 possible to specify a passphrase when generating the key which
791 will be used to encrypt the sensitive part of this file using
798 Contains the public key for authentication. These files are not
799 sensitive and can (but need not) be readable by anyone.
802 Contains a list of host keys for all hosts the user has logged
803 into that are not already in the systemwide list of known host
804 keys. See sshd(8) for further details of the format of this
808 Commands in this file are executed by ssh when the user logs in,
809 just before the user's shell (or command) is started. See the
810 sshd(8) manual page for more information.
813 This file is for host-based authentication (see above). It
814 should only be writable by root.
817 This file is used in exactly the same way as hosts.equiv, but
818 allows host-based authentication without permitting login with
822 Systemwide configuration file. The file format and configuration
823 options are described in ssh_config(5).
825 /etc/ssh/ssh_host_key
826 /etc/ssh/ssh_host_dsa_key
827 /etc/ssh/ssh_host_ecdsa_key
828 /etc/ssh/ssh_host_rsa_key
829 These three files contain the private parts of the host keys and
830 are used for host-based authentication. If protocol version 1 is
831 used, ssh must be setuid root, since the host key is readable
832 only by root. For protocol version 2, ssh uses ssh-keysign(8) to
833 access the host keys, eliminating the requirement that ssh be
834 setuid root when host-based authentication is used. By default
835 ssh is not setuid root.
837 /etc/ssh/ssh_known_hosts
838 Systemwide list of known host keys. This file should be prepared
839 by the system administrator to contain the public host keys of
840 all machines in the organization. It should be world-readable.
841 See sshd(8) for further details of the format of this file.
844 Commands in this file are executed by ssh when the user logs in,
845 just before the user's shell (or command) is started. See the
846 sshd(8) manual page for more information.
849 ssh exits with the exit status of the remote command or with 255 if an
853 scp(1), sftp(1), ssh-add(1), ssh-agent(1), ssh-keygen(1), ssh-keyscan(1),
854 tun(4), hosts.equiv(5), ssh_config(5), ssh-keysign(8), sshd(8)
856 The Secure Shell (SSH) Protocol Assigned Numbers, RFC 4250, 2006.
858 The Secure Shell (SSH) Protocol Architecture, RFC 4251, 2006.
860 The Secure Shell (SSH) Authentication Protocol, RFC 4252, 2006.
862 The Secure Shell (SSH) Transport Layer Protocol, RFC 4253, 2006.
864 The Secure Shell (SSH) Connection Protocol, RFC 4254, 2006.
866 Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints, RFC
869 Generic Message Exchange Authentication for the Secure Shell Protocol
870 (SSH), RFC 4256, 2006.
872 The Secure Shell (SSH) Session Channel Break Extension, RFC 4335, 2006.
874 The Secure Shell (SSH) Transport Layer Encryption Modes, RFC 4344, 2006.
876 Improved Arcfour Modes for the Secure Shell (SSH) Transport Layer
877 Protocol, RFC 4345, 2006.
879 Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer
880 Protocol, RFC 4419, 2006.
882 The Secure Shell (SSH) Public Key File Format, RFC 4716, 2006.
884 Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer,
887 A. Perrig and D. Song, Hash Visualization: a New Technique to improve
888 Real-World Security, 1999, International Workshop on Cryptographic
889 Techniques and E-Commerce (CrypTEC '99).
892 OpenSSH is a derivative of the original and free ssh 1.2.12 release by
893 Tatu Ylonen. Aaron Campbell, Bob Beck, Markus Friedl, Niels Provos, Theo
894 de Raadt and Dug Song removed many bugs, re-added newer features and
895 created OpenSSH. Markus Friedl contributed the support for SSH protocol
896 versions 1.5 and 2.0.
898 OpenBSD 4.9 November 18, 2010 OpenBSD 4.9
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